So- what actually happened?For the purpose of this the only sources we will consider canon are the 26 episodes of the original anime and I will talk about the movie ?End of Evangelion?. There is a lot of mythos so here is the background, in a nutshell. I?m going to be talking in super simplified strokes and just know that there is some debate over some specifics that I won?t get into:
What You?ve Gotta Know
Souls and bodies are independent from one another. A soul can make its own body from a special substance (think of it like the primordial ooze) or can inhabit a vessel made for it.Many thousands of years ago there is an alien race was on the brink of destruction. They detached their souls from their body, stored them in these ships, and made some beings that would go to other planets and populate them with vessels that this alien race could be reincarnated in. These beings are called the ?Seeds of Life?. (Still with me?)
Two of these ships land on primordial earth. The first Seed of Life, Adam, arrives. The second Seed of Life, Lilith, arrives shortly after. Each of these ships are accompanied by a self defense weapon. In the series they are referred to as the ?Lance of Longinus? by humans (they look like spears). Lilith?s is damaged or destroyed in her impact with Earth. There is a rule in their programing that only one Seed of Life can be active on a given planet so the Lance of Longinus automatically incapacitates Adam. Lilith starts producing offspring and that?s basically all life on Earth. Eventually Adam?s body is discovered in Antarctica. They remove the spear and he (it?) starts to wake up. Someone tries to ?connect? with it. While there is some debate, this basically means shedding their physical form and merging their soul with Adams. It doesn?t go so well- Adam begins to explode- they try to shove the spear back in but it?s too late. Antarctica explodes- wiping out all life in a huge radius (down to the microscopic level). This is that ?Second Impact? thing they are talking about. As the result of the explosion Adam?s body and spirit were separated. The body is that gross fetus thing that Ryoji (the scruffy looking spy guy) sneaks out. It?s spirit goes into a vessel (later we call him ?Kaworu?) and SEELE controls both. In simple term the angels, the children of Adam, start being born and seek to wipe out humanity and Lilith.
This group called ?SEELE? has this plan. They basically got ahold of the instruction manual from one of the Seeds of Life?s ships. They call this the ?Dead Sea Scrolls?. They understand, more or less, what?s going on. They hatch this plan- they want to have all of humanity shed their physical form and join together with Lilith. This is that ?Human Instrumentality Project? they talk about. ?Instrumentality? is the term they use to describe ?all of humanity as souls, merged together?. It should be noted that it is also possible that SEELE just wanted ITS members to join together in an Eva and become a god. Evas are all clones of Adam except Unit 01, Shinji?s Eva. It is actually a clone of Lilith. They do not have a soul- they are a vessel for a soul. Remember how I said souls and bodies exist independent of each other? So each of the Evas are GIVEN a soul by a ?contact? method. Yuki Ikari (Shinji?s mom) is in Unit 01, Asuka?s mom is in Unit 02 (well- part of her soul. That?s why her mom is crazy). It?s not clear who is in Unit 00?s but it?s suggested that it might be Rei (the first Rei, who dies as a child). This is why all the characters can only pilot their own Evas. They are basically connecting with their mom?s souls.
Let?s talk about Rei.She?s a WEIRD one. So Shinji?s mom, Yuki Ikari, has her soul pass into Unit 01. The remains are basically cobbled together into a vessel. The FIRST Rei (there are lots of clones) is probably the leftover bits from Yuki. The later ones are just vessels for souls. The later ones have Lilith?s (the Seed of Life) soul in it (or part of it). Earlier I mentioned a ?special substances? that substance that a soul can make a body out of. If that soul is born from Lilith that stuff is called ?LCL? in the series (it?s the breathable liquid in the entry plug). If it?s a soul from Adam it?s called ?Particle-Wave Matter? (PWM). That?s what the angels are made out of. It?s kinda gross but NERV harvests the blood of Lilith to get LCL. So now we get to the actual series. Angels (Adam?s kids) are trying to wipe out humanity and Lilith so they can flourish as intended originally. They want to kill Lilith, which is locked up in the lowest level of NERV headquarters so she can?t make any more kids. This is why Tokyo 3 keeps getting attacked. Shinji, piloting an Lilith based Eva, fights bunch of Angels and kills them.
Something about Adam and Lilith.Apparently there were two ?types? of Seed of Life. Adam is a ?Fruit of Life? type and Lilith is a ?Fruit of Knowledge? type. That ancient alien race split them up because if the children from a Fruit of Life and Fruit of Knowledge type were ever combined- they?d be as strong as that ancient alien race (basically- a god). The ?Fruit of Knowledge? is basically ?the power to do science and technology? and is what humanity has. The ?Fruit of Life? is basically near-unlimited power and that?s what the Angels have. A lot of things happen but the important bit is that Eva Unit 01 (remember- Lilith based) takes this organ from an angels and eats it. This is the ?Fruit of Life?. They refer to it as the ?S2 engine? in the series (they call a lot of biological things mechanical names- it?s an organ). So Unit 01 now basically has: Yui?s soul in it, parts of Lilith (Fruit of Knowledge), and parts of Adam (the ?Fruit of Life?). This basically makes it a nascent god (power on par with that super powerful ancient alien race). A note on ?AT Fields?.Basically these are the barrier to your soul (remember- soul and vessel are independent). An ?AT Field? is basically what holds a soul in its body and keeps a vessel looking like what it looks like. Think of it like a? self image. Adam has an ?Anti-AT Field? which can free people?s souls from their vessels. Eva Unit 01, once it has eaten that S2 engine, can now basically do an Anti-AT Field.
Let?s talk about Kaworu Nagisa for a moment.Basically- he?s Adam?s soul in a different vessel. They call him the ?17th Angel? but he?s not really an ?angel? per se. (Adam and Lilith are basically race making machines called ?Seeds of Life?.) He shows up because, SEELE who have been holding him, wants to initiate their ?Human Instrumentality Project? (remember- free all souls, reunite humanity, etc). I?m skipping over some stuff but they need Unit 01 to serve as the catalyst for it. So Kaworu gets to know Shinji. He goes down into NERV to try to free Adam, finds out they have Lilith and not Adam. Basically if Kaworu merged with Lilith, humanity would explode and everyone would die. Shinji grabs him using his robot and Kaworu decides to not kill all of humanity because it?s a good race. He asks Shinji to kill him so he can?t be used to destroy the human race. Cue music, squish, blood, scream, roll credits.
The last two weird episodes are really just going on in Shinji?s mind. Basically, just ignore them. They have some impact but? not really.
Let?s talk about the movie ?End of Evangelion?.Basically what happens here is: The Human Instrumentality Project happens anyway because of some stuff resulting in the ?Third Impact?. (Basically, Adam and Lilith finally got together because Gendo shoved the Adam fetus in Rei.) This is the ?freeing of all the souls of Lilith-based lifeforms? (humans). Shinji in Eva Unit 01, who was the catalyst for all this, is spared and asked by the new god (it?s a weird? Rei, Lilith, kinda Adam hybrid) if he wants all of humanity to be merged. He explores his feelings on the matter and decides it would be best if everyone was an individual. This doesn?t do much as all of humanity has already left their body and ?died? so it?s basically just him and Asuka on a beach on a planet devoid of life. The implication is that, eventually, the rest of humanity?s soul?s will probably be born into new vessels at some point.
So yeah- there you have it.
Basically, Evangelion is a story about two aliens who come to populate the same planet by accident with the souls of their dead friends by making vessels for them to be reincarnated in. They fight, Shinji whines a bit, they merge, they decide the whole thing is silly and call it off.