Here's the official synopsis:
"The Invincible Iron Man and the Incredible Hulk join forces to save the Earth from its greatest threat yet! When two Hydra scientists try to supercharge a Stark Arc Reactor with Hulk's Gamma Energy, they unleash a being of pure electricity called Zzzax - and he's hungry for destruction. Together, Iron Man and Hulk are the only force that stands in the way of the Zzzax's planetary blackout. But first the super heroic duo will have to get through snarling Wendigos, deadly robots and the scaly powerhouse, Abomination. Can two of Marvel's mightiest heroes find a way to work together without smashing each other before time runs out?"
Featuring in the voice cast for Iron Man & Hulk: Heroes United is Adrian Pasdar (Heroes, Ultimate Spider-Man) as Iron Man and Fred Tatasciore (Wolverine and the X-Men, The Avengers: Earth's Mightiest Heroes) as The Hulk. The script has been written by Henry Gilroy (Star Wars: The Clone Wars) and Brandon Auman (Iron Man: Armored Adventures), while Eric Radomski (Batman: The Animated Series) serves as Supervising Creative Director. You can watch the trailer here.