Wesley Gibson (James McAvoy) trains to be an assassin to kill Cross (Thomas Kretschmann), who Sloan (Morgan Freeman) says killed Gibson's father. Fox (Angelina Jolie) is his main trainer.
Turns out Sloan is the bad guy and Cross is Gibson's father. Gibson finds this out after he has fatally shot Cross.
Gibson tells the rest of the assassins that Sloan lied to them, and they were all fated to die. Sloan tells them to kill Gibson, but Fox shoots a curved bullet that goes through the heads of all the assassins, including herself. Sloan escapes.
The scene then returns to what appears to be Gibson back at his old job. Sloan appears, pointing a gun to his head. Gibson turns around, and it's not really him - it's a decoy. A bullet slams through Sloan's head. We then re-trace the bullet's winding path backwards, just like a scene from the beginning and see that it was Gibson who shot it, just like his father had done.