"Things go very wrong when Tony Stark and Dr. Bruce Banner begin work on a global peacekeeping system, putting Earth's Mightiest Heroes--Iron Man, Captain America, Thor, the Incredible Hulk, Black Widow, and Hawkeye--to the ultimate test. The evil Ultron threatens the fate of humankind, and it is up to the Avengers to stop his terrible plans. Uneasy friendships and unexpected action make for one exciting, epic adventure"--Back cover.