The death of the Kelvin timeline's James Kirk from radiation in 2259 while restarting the Enterprise's warp core, and his revival with Khan's blood.
The event comes at the climax of Star Trek Into Darkness. After the fight with the USS Vengeance, the Enterprise is falling towards Earth with her warp core out of alignment. Kirk knocks out Scott, enters the radiation-filled chamber and kicks the core back into place. Power returns and the ship is saved.
Spock reaches him as he lies dying behind the sealed door, a scene that reverses their roles from Star Trek II: The Wrath of Khan. Spock then chases Khan through San Francisco in a rage.
McCoy has meanwhile seen a dead tribble revive after being injected with Khan's blood. Uhura stops Spock from killing Khan, since he is needed alive, and McCoy uses the blood to make a serum. Kirk wakes about two weeks later.