okay so SPOILERS....obviously -
Cumberbatch is KHAN and he is one of two villains - and kind of the lesser - the real Monster here is ADMIRAL MARCUS.
It's only when KHAN fails to escape WITH his people [still in cryo] that he turns really nasty.
The basic rundown is that Marcus and Section 31 [I love to hate those guys] search for and find KHAN and his crew - they awaken KHAN and force him to help them design weapons so that they can PROVOKE a war with the KLINGONS.
MARCUS is building a Warship in secret that is much larger than the enterprise - can travel faster warp speeds - can fire while at warp!!! - can't recall if they actually reference the ship by name in the movie.
The ending of the Film borrows heavily from TWOK but with roles changed around. Kirk dies of radiation - Spock screams out "KHAAAAAAAAAAN" - oh, and they still outsmart him with some quick thinking.
Prime spock [Nimoy] makes an appearance. He tells Quinto-Spock that he doesn't want to interfere - then tells him how dangerous Khan is - then doesn't bother to tell him ANYTHING USEFUL in defeating that man. Pointless cameo was pointless.
There is mention of New Vulcan but we don't get to see it.
Khan teleports from EARTH to KRONOS [english spelling] using Scotty's site to site algorithm from the previous film.
CAROL MARCUS lies about her identity to slip aboard the enterprise - she suspects her father is up to something.
In the end they just put KHAN back into cryo in a warehouse and set out on the 'five year mission'
Despite all that I really enjoyed the film. It plays a lot better onscreen than it reads in text.