Normally this sort of thing comes with the trailer, but now we finally have an official synopsis for Batman V Superman: Dawn of Justice.
Collider attended a licensing show in Las Vegas, and spotted this synopsis for the film:
“Fearing the actions of a god-like super hero left unchecked, Gotham Cityâ€™s own formidable, forceful vigilante takes on Metropolisâ€™s most revered, modern-day savior, while the world wrestles with what sort of hero it really needs. And with Batman and Superman at war with one another, a new threat quickly arises, putting mankind in greater danger than itâ€™s ever known before.”
This is basically what we see in the first trailer for the movie. The new threat that arises is rumored to be Doomsday. According to rumors, Lex Luthor creates Doomsday from the corpse of General Zod. He also modifies Zod’s armor to be his familiar green power armor from the comics.
Luthor creating Doomsday actually has a thread from the comics as in the New 52 Luthor created a Bizarro as a failed Superman clone. So having Doomsday be some twisted clone of Zod does fit in with what the comics have been doing lately.