Nothing, nobody but apocalypse dies.
The plot is:
Apocalypse is the first mutant and has the power to transfer his mind into another body when his current body is too old and weak, he gains the powers of every mutant he does this too, so he's like a super-mutant with tons of powers. He is trapped underground in Egypt for centuries until the 80's where the movie is set.
Magneto is living in Poland as a steelworker under a new identity when he is found out and his wife and daughter are killed by police. This makes him join Apoc when he returns as Apoc wants to wipe out all non-mutants from Earth. Apoc has the power to super-charge any mutants power which he does to Magneto, Storm, Angel and Psylocke who are his four horsemen.
Mystique finds Nightcrawler.
Cyclop's boring backstory and character is introduced.
It causes Storm to have white hair and the super powerful powers we know her to have.
Apoc causes havoc on Earth with the powers of his followers. He kidnaps Prf. X, and X mansion blows up, Quiksilver is Magnetos son.
The rest of the Xmen are brought to the weapon X facility where wolverine makes a brief cameo (he kills some guards and escapes into the woods.) The Xmen are broken out by Jean Grey, cyclops and Nightcrawler.
Apoc wants to use Prof. X's body to transfer into, the rest of the X-men travel to his newly made pyramid in Egypt and stop him. Apoc's body swap thingy gets interrupted mid-way and fails, causing Prof. X to lose his hair.
There is a big fight, Angel and Psylocke (I think) are killed, Storm and Magneto turn against Apoc after being convinced to be good again. They all attack him, them Prof. X and Jean Grey combine their psychic abilities to take Apoc down.
They rebuild the mansion, mystique survives, although she was almost killed.
The after-credits scene is some corporation called "Essex Corp" collecting mutant DNA from Strykers facility and putting it in a briefcase.
The End.