What is Fate:
Explaining what Fate is is not something that can be done quickly and easily; however the basics of it really aren't that complex. The Fate series originated as a visual novel created by Type-Moon, and was written by co-founder Kinoko Nasu. It was based off of a novel idea he had, and was then altered into the series we know today. It takes place in a different timeline than that of his earlier works, but carries over a lot of the same themes and ideas that were established in works such as The Garden of Sinners, and Tsukihime. That being said these series are not needed to follow Fate, but long time fans of Nasu's work will already be familiar with some things because of this. Sometimes characters from those other timelines are seen or mentioned, and you have groups like the Mage Association who work at Clock Tower in London. This is the same group across all timelines and worlds, but again, they are different timelines with vastly different stories.
As for Fate itself, the first entry is Fate/Stay Night (which was originally released for PC), and all future Fate media use this game as a base for it's worlds. Sure there are alternate timelines to Stay Night as well, but those are side stories created after the fact. They are there to expand the universe, and offer something different. The main timeline consists of only a few entries, with it all "beginning" at Zero. But here is where the problems lie.
Zero is OUR Beginning:
Fate/Zero was a prequel novel written by Gen Urobuchi, and as such it's written for fans who are already familiar with Fate/Stay Night. It explains a lot of the events that lead up to Stay Night, and focuses on characters who would go on to play key roles throughout the series. Some of these characters were originally mentioned in Stay Night, but didn't have much screen time keeping a lot of their lives a mystery. Meanwhile other characters were front in center throughout Stay Night's story, but their motives and past experience would not always be so clear. Zero is what gave fans a look back into their lives to see exactly how things ended up the way they are at the start of Stay Night, and gives us a much better insight to the world itself. Going into Zero fans will already recognize most of the lead staff, and understand who they will eventually become. It's this knowledge that helps keep you hooked from start to finish, with some truly shocking revelations spread throughout the entire story. The problem is we here in the West don't really get to experience it this way.
Considering Stay Night was never released outside of Japan, and that we only have a messy anime adaptation, the anime version of Zero ultimately is our only real good starting point. As it's a prequel we can jump in with little problem, but it takes awhile before you become invested in it. At first you don't have a reason to care about these characters, and it can be a little hard to follow; however if you stick with it things do fall into place, and by the end you'll see just what makes this series so great. And then once you continue on with the series, you can always come back for a rewatch so that you can truly appreciate all that Zero is. So yes, we can get to the same point as Japanese fans, but it does take some time.
The Holy Grail War:
The story of Fate/Zero focuses around the events of what is known as the 4th Holy Grail War. These "Wars" come from a system mage families of the past set up years ago, and sees the Holy Grail itself being summoned to grant one person's any wish. However choosing that one person is not that simple. As the Grail itself has great power, the Grail is used to choose it's owner. It selects multiple "Masters" to take part in it's war, and then grants them the power to summon a legendary hero to fight by their side. Master and Servant will then partake in a battle to the death, with the last person standing being the winner. It's a straight forward concept, but the way things work actually gets a little bit more complex.
In the world of Fate everyone has what is known as an Origin. These "Origins" are what everyone world's soul originally began as, and is what defines who they are. Origins come from the center of all creation, a place known as the "Root," and is something many mages in the world strive for. With the use of the Holy Grail many believe they can finally reach the Root, but being selected to take part in the Holy Grail War is not a simple process. Only mages are allowed to enter this fight, and who a mage is is determined completely by birth. A mage is not the same as a witch by normal standards, and is actually a lot more similar to a super hero with special powers. It's something families can pass on to their offspring, and every mage in the world is born with a different type of power. This is how the world of Fate works, and what determines if someone can be in the Holy Grail War or not.
As for how the Holy Grail summoning system works, it runs completely off of a location known as the "Throne of Heroes." This is a location beyond time and space, and serves as a record of the people living in all timelines. When someone meets the criteria to be chosen as a "Heroic Spirit," all of that person's data is recorded by the system, and a Heroic Spirit is produced from it. This is where things get a little messy though. As the Throne of Heroes is a type of record for the world itself, the world has an impact on it also. Legends mix with reality, and a Hero Spirit is effectively changed. Just because real history tells us one thing about someone, the thousands of made up stories about them told world wide will be there to overwrite the facts. If it's rumored someone had a special power and the knowledge becomes wide spread, then that Heroic Spirit will in fact gain said power. This also means some Heroic Spirits will not be real people at all, but instead creations of the people. Or could it be they were real, and that the stories were simply based on them? This is a question that is never clearly answered in Fate, but is brought up constantly. Either way the fact remains that Heroic Spirits can be either our real world historical characters, or those from legend.
Understanding the Key Characters of Zero:
So once you have an understanding of the Holy Grail War (which really just comes down to mages using Heroic Spirits to fight for the Holy Grail), the next major thing about Zero is the characters themselves. Again long time fans will already know who many of these people are, but newcomers will be going in blind and may have a hard time following them. That being said I hope to clear up the confusion now, and make this story so much easier to get into. This is something I've explained for friends in the past with success, so bear with me.
Emiya -
The first major character we come across is Kiritsugu Emiya. Kiritsugu is a mercenary who has had a very hard life. By this point in the story he has been through multiple tragic events, and had to kill many people due to his line of work. The world sees him as a monster who is willing to do anything to take out his target, but the truth of that is a lot more horrific than one would expect. The things Kiritsugu has done might have been of his own volition, but they are decisions someone should never have to make. The life he has lived continued to eat away at his soul, and he eventually came to one conclusion. If he wants to change the world for the better, and truly help people, the only way to do so is by using the so called Holy Grail. This is why Kiritsugu decides to enter the War himself, but it's not something he can do alone.
Kiritsugu joins the Einzbern family in their quest for the Holy Grail. This family has a long standing history with the Holy Grail War, and their artificially created humans are key in the Grail's creation. One of which is a woman by the name of Irisviel. Her purpose is to be used for the Holy Grail War, and there's no way around this. The war will lead to her death, but knowing this doesn't stop Kiritsugu from falling in love with her. At the start of the series we see the birth of their daughter Illyasviel, but this event is a far cry from the happy occasion it should be. Kiritsugu and Irisviel love their daughter, but they know if they fail to save the world and end the Holy Grail War here, then she is the one who will eventually pay the price.
Along with Kiritsugu's long time partner Maiya, these three make up the first main group of characters in Fate/Zero. They get together and summon King Arthur, who they realize is actually a woman by the name of Artoria Pendragon (now localized as "Altria" in other Fate media). She is a part of the Saber class, and thus prefers to be called "Saber" to hide her true name. Something all Heroic Spirits tend to do to protect their identity. Artoria herself has a strong sense of justice and doesn't agree with Kiritsugu's "do whatever it takes to win" mentality, but she has no choice but to fight for her master. As every master is given what is known as "Command Seals" (later media translates this as "Command Spells"), Kiritsugu can force her to do anything he wants her to at least three times -- very much to her dismay.
Kotomine and Tohsaka -
Kiritsugu isn't the only person to team up for the Holy Grail War. The Kotomine family has been a part of the chuch for many generations. While the church normally looks down upon things such as magecraft, they are ultimately the ones in charge of overseeing the Holy Grail War. While the Grail itself is not actually the same Grail from Jesus' time, it's still seen as a relic of the church. Kirei Kotomine is a broken man who is forced into the war after a Command Seal appears on his hand. His father decides to team him up with a man named Tokiomi Tohsaka, and sends him off to Japan to prepare. While his father thinks this will be a good distraction for Kirei after the tragic death of his wife, the reality is... Kirei just doesn't care. Again, Kirei is a broken man, and it's something that becomes painfully more obvious as the series progresses.
Near the start of the series Kirei has no ambition, and honestly doesn't care what happens. In fact he actually finds pain to be pretty interesting. He goes along with Tokiomi because he's told to, and takes part in the war without caring who wins or loses. He himself summons an unknown Assassin class servant, while Tokiomi takes the Archer class. This Archer however is anything but your typical servant, as he is none other than the King of Heroes himself -- Gilgamesh. It's Gilgamesh who will eventually put Kirei on his own path in life, and who will become a major player across the entire Fate series.
Matou and Tohsaka -
The Matou family is a group of people who serves as a major adversary throughout all of Fate, and most of what they do all begins here. As a family that is slowly losing it's mage bloodline, the family head comes up with a plan to carry on it's legacy, and eventually obtain the Holy Grail for themselves. This is where Kariya comes in.
Kariya Matou was to be the one to carry on the family's torch. He was arranged to marry a woman named Aoi, and was expected to produce children with a greater magical potential. The problem was though that Kariya truly did love Aoi, and because of that he couldn't stand knowing the fact that old man Matou planned to use his children as tools. To avoid this Kariya abandons his family, and leaves Aoi to protect her and her future. Except things don't go as planned.
After Kariya left, the head of the family instead arranged Aoi's marriage to Tokiomi Tohsaka. With the Tohsaka being another powerful mage family, the marriage to Aoi would be beneficial to them as well. Any offspring they produced would have greater magical power, and would continue their families abilities into the next generation. As a part of this arrangement however, old man Matou required one thing. The two's second born.
Jump ahead to the start of Fate/Zero and we see Kariya visiting Aoi and her two daughters Rin and Sakura. Despite not being a part of the family, Kariya still loves Aoi and acts as an uncle to her children. However on this day things are different. Rin is there, and happy to see her uncle, but Sakura is no where to be seen. As it's getting closer to the Holy Grail War, old man Matou finally made his move. Sakura became his.
Angered at this fact, Kariya returns home, and witnesses a horrifying sight. Sakura being consumed by magical creatures known as "Crest Worms," which in return will boost her magical power even more. Being unable to stand knowing what will happen to Aoi's children anyway (despite him originally leaving Aoi to prevent this), Kariya agrees to take young Sakura's place in the war, and undergoes the treatment instead. This is how Kariya gets involved with the 4th War, and he summons a maddened Berserker class servant to fight for him. He hopes to win the Holy Grail to not only save Sakura, but to take revenge on Tokiomi for allowing this to happen as well.
Waver and El-Melloi -
The next (and final main) group in Zero is actually two smaller ones that are connected. Kayneth El-Melloi is a teacher at the Mage Association in Clock Tower, and Waver Velvet is his student. Waver is a young mage from a family that only just developed it's abilities, and because of that he is looked down upon by other higher ranking mages. He wants to prove himself to be something greater, but even his teacher El-Melloi doesn't think much of him. One day after class Waver catches wind of "something" going down, and uncovers the secret of the Holy Grail War for himself. Realizing his teacher is about to take part in it, Waver breaks into his room, steals the relic his teacher planned on using to summon his servant, and uses it for himself once he manages to be chosen by the Grail. This relic summons none other than the Rider Alexander the Great himself, and forces his teacher Lord El-Melloi to summon a Lancer instead. El-Melloi is joined by his soon to be wife, while Waver only has Alexander to fall back on.
The thing about Waver is... He's a stupid kid. He's gotten himself into something he can't really comprehend. His personality clashes with his servant's, and he seriously doesn't know what to do. He can't fight, he's scared of conflict, and he's very hard headed. He wants to do things his own way all the time, but he's too naive to listen to. Alexander (aka Iskandar) realizes this and ultimately does his own thing whenever possible, but the fact remains that Waver is his master. Throughout the series both characters do have an impact on each other, with Waver ultimately being changed for the better.
Blue Beard -
The real final group of Fate/Zero is the odd man out. Ryuunosuke is a crazy serial killer, and that's about as much as you need to know. He kills people for the fun of it, and while doing so is somehow chosen by the Grail to fight in the war. After slaughtering a family he uses their blood to summon none other than Gilles de Rais, and the two go on a mass killing spree for the fun of it. They aren't fighting in the war, they are just doing whatever it is they want. However, this cannot be tolerated.
The Battle of Tactics:
Unlike most battle royale style "killing games," the war in Fate/Zero is not the same. Instead of out right attacking each other, and battling to the death, it's more like a game of chess. Each person in the war wants to win it, but to do so they need to outsmart the other players. Considering they only know basic information about each other, and their servant's class name, their actual skills and abilities are unknown. Simply running into battle without a plan is a great way to get themselves killed, so critical thinking and trickery becomes key to survival.
Throughout the series we get to learn more about each participant in the war, and we also get to see their plans get laid out and eventually put into action. Kiritsugu has Maiya helping him in the shadows, so she already puts him at an advantage. Kotomine's servant is unique in multiple ways, and the fact that he's working with Tokiomi has remained a secret. Waver gets to see a lot of events unfold from the sidelines as he learns more about the world and himself, and his teacher is able to use his many connections to research the others involved in the war. Kariya has power on his side, and has his targets set on Tokiomi, while crazy serial killer has "fun" in the background until he can't go unnoticed any longer. Everyone has their own goals, and by constantly switching between the characters we get to see the grand scheme of things. We get to see what one character is planning in advance, and then the results of his actions as the others try to deal with it. Then you have other characters setting up their moves well in advance, with those plans coming into play exactly at the right moment. It's a battle of wits between multiple parties, and as the viewer you get to understand it all and see exactly how things fall into place. Waver himself is in this very same situation, but he is powerless to do anything about it. It doesn't take him long to realize he has little hope of winning the war, but he is smart enough to understand what others have done to work towards that goal. This is something that will eventually come back into play when Fate/Zero is revisited... But that's a story for another time.
Although it takes awhile for Fate/Zero to get going, once it does it turns into a roller coaster ride that never stops. The first half of the series takes it's time to set up the pieces in this chess game, while the second half is where all of these plans come into play. At this point characters are forced to constantly change and adapt to their new situations, and only those will the most knowledge can come out on top. While some characters will prevail and typically come out on top, others find themselves being manipulated into doing exactly what the others want. What it ultimately comes down to is, is the plan they are carrying out really a plan of their own accord? Or are they actually being used by someone else? Sometimes the answer reveals itself after it's already too late.
Should You Watch It:
The real question is, why SHOULDN'T you watch it? Really if you'll like the series or not all depends on your own taste. If you're into mind games/battles of wit, battle royales, and crazy dramatic storylines with tons of plot twists with a great cast of characters... Then there's a really good chance you'll love Fate/Zero. It does have a slow start and is a bit hard to follow at first, but that's mainly because as an anime adaptation of a novel series, there's only so much that can be covered. Also mix in the fact that Zero was created with people who already played Stay Night in mind, there's other things that may simply go over newcomer's heads right off the bat. That's why I took the time to explain the basics here, as well as who these characters are. There are specific parts of each character's story that are glossed over in Fate/Zero, but with this post I basically filled in most of those blanks. Other parts of the story really does know Fate/Stay Night knowledge to fully grasp, but for the time being it's fine. Overall Zero does work pretty well as a prequel to the series, but it's really something you'll want to return to later on. On another note, the animation by Ufotable is simply amazing here, and it's easily one of the most detailed anime series to date. Of course Ufotable has only gotten better since the release of Zero, but Zero still outranks most other series on the market. There's a reason the follow up "Fate/Stay Night Unlimited Blade Works" is often called "Unlimited Budget Works," and that all really began here.
That being said, you do have to understand that Zero is just the intro to the greater Fate storyline, and it leaves things off in a way that gets viewers ready for what come's next. It does tell a complete story of it's events and characters, but you really don't want to stop there. Trust me. The Fate series is well worth the time and effort it takes to get into, with even the low points of the series still being entertaining in their own way. Of course those series also take place on different timelines, so it's perfectly understandable if you choose to ignore them and only focus on the "main" path. So yeah, what are you waiting for? Go give it a chance! Even if it doesn't hook you from the get go, push through and wait until part two to judge it. You'll thank me later. (Or not.)
