A cosmic entity called the Beyonder observes the mainstream Marvel universe. Fascinated by the presence of superheroes on Earth and their potential, this entity chooses a group of both heroes and supervillains and teleports characters against their will to "Battleworld", a planet created by the Beyonder in a distant galaxy. This world has also been stocked with alien weapons and technology. The Beyonder then declares: "I am from beyond! Slay your enemies and all that you desire shall be yours! Nothing you dream of is impossible for me to accomplish!"
The heroes include the Avengers (Captain America, Captain Marvel II, Hawkeye, Iron Man, She-Hulk, Thor, the Wasp); three members of the Fantastic Four (Human Torch, Mister Fantastic and Thing); solo heroes Spider-Man, Spider-Woman and the Hulk; and the mutant team X-Men (Colossus, Cyclops, Nightcrawler, Professor X, Rogue, Storm, Wolverine and Lockheed the Dragon). Magneto is featured as a hero, but immediately becomes non-aligned when the Avengers question his presence.
The villains include the Absorbing Man, Doctor Doom, Doctor Octopus, the Enchantress, Kang the Conqueror, Klaw, the Lizard, Molecule Man, Titania, Ultron, Volcana, and the Wrecking Crew. The cosmic entity Galactus also appears as a villain who immediately becomes a non-aligned entity.
The heroes (the X-Men choose to remain a separate unit) and villains have several skirmishes. There are several significant developments in the series: villainesses Titania and Volcana are created; the second Spider-Woman, Julia Carpenter, is introduced; Spider-Man finds and wears the black costume for the first time, initially unaware that it is actually an alien symbiote (the symbiote would subsequently bond with journalist Eddie Brock, giving birth to the villain known as Venom); Doctor Doom temporarily steals the Beyonder's power; having fallen in love with the alien healer Zsaji (who sacrifices her life on Battleworld to save the heroes), mutant Colossus ends his romantic relationship with a heartbroken Kitty Pryde; and the Thing chooses to remain behind on Battleworld and explores the galaxy for a year, with She-Hulk temporarily joining the Fantastic Four as his replacement.
Years later, it was revealed that -- while the heroes and villains fought on Battleworld -- the Thing's girlfriend, Alicia Masters, was replaced by Lyja, a Skrull spy. When the Fantastic Four returned to Earth without the Thing, she began a relationship with the Human Torch. The two eventually married, and remained a happy couple until the Skrull's true identity was exposed by the Thing and the Puppet Master.
- Xander Berkley - Tony Stark/ Iron Man, Beyonder
- Clancy Brown - Ben Grimm/ The Thing
- Rino Romano - Steve Rogers/ Captain America, Cyclops
- Cree Summer - Ljya, Zsaji, She Hulk
- Andrea Baker - Alicia Masters
- Jason Marsden - Spider-Man/ Peter Parker, Colossus
- Will Friedle - Johnny Storm /Human Torch, Bobby Drake/ Iceman
- Mark Hamill - Victor Von Doom/ Dr. Doom, Ultron
- Kevin Michael Richardson - Absorbing Man, The Hulk
- Jonathan Freeman - Kang the Conqueror
- Corey Burton - Reed Richards/Mr. Fantastic, Odin, Dr. Octopus, Puppet Master
- Nicole Sullivan - Susan Storm-Richards/ Invisible Woman
- Sumalee Montano - Titania, Volcana, Julia Carpenter/ Spider-Woman
- Ashley Johnson - Rogue
- Ron Perlman - Magneto, Klaw
- Jeff Bennett - HERBIE, Molecule-Man, Thor
- Andre Soliquizo - Puppet Master
- Hynden Walch - Kitty Pride/ Shadowcat
- Dee Bradley Baker - Lockheed, Lizard, Skrulls, Nightcrawler
- Katja Zoch - Volcana
- Troy Baker - additional voices