Marvel's Iron Man & Hulk: Heroes United is an animated movie featuring a team up between Tony Stark and Hulk (who never assumes non-Hulk Bruce Banner form in the movie). It's not exactly a buddy movie. Despite the "Heroes United" tag line - these guys don't really get along. In fact, one of the best parts of the movie is a fight between the two Avengers.
Their real opponent, though, is a living energy force known as Zzzax. Zzzax is the product of a Hydra experiment involving a combination of an Arc Reactor and Gamma radiation. (Which, you may note are the sources of Iron Man's and Hulk's powers respectively.) In fact, Hulk is attacked and kidnapped by Abomination who brought him to Hydra for the experiment.
It isn't exactly clear (at least to me) what Hydra was trying to do, exactly, because they were shocked by the results and ran off.
Zzzax's only desire and motivation is the consumption of energy. He says that humans are wasting it, (a tiny bit of policizing there), and he needs to have it all. (Which, sounds like a waste to me.) So in order to stop a global black out, the two bickering powerhouses team up to battle this being of pure electricity.
The movie is pretty good. Some of the jokes are cringe worthy, the art style takes a bit of getting used to, and it seems to me they drug out some of the scenes to get to the "Feature Length Presentation" 72-minute status. But it's a fun ride overall.
It is available on Blu-Ray, DVD, and digital download and various combo packs wherever comic-book movies are sold.