Thanks to the Licensing Expo in Las Vegas, Collider got their hands on the official plot synopsis of Batman v Superman: Dawn of justice:
Fearing the actions of a god-like super hero left unchecked, Gotham City’s own formidable, forceful vigilante takes on Metropolis’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 it’s ever known before.
I don’t think anyone is suprised by the synopsis. They fight, they team up. Pretty standard fare.