A godlike cosmic entity called the Beyonder has been observing the Earth-616 universe - the mainstream Marvel universe. Fascinated by the presence of metahumans on Earth and their potential, this entity chooses a group of both superheroes and supervillains and teleports them (against their will) to "Battleworld", a planet created by the Beyonder in a distant galaxy, and stocked with alien weapons and technology. The Beyonder then issued the following decree: "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 (the X-Men choose to stay as a separate unit) and villains have several skirmishes, and there are several significant developments both within the series:
- Spider-Man finds and wears a black costume for the first time. He is initially unaware of the fact that the costume is in fact an alien symbiote and will eventually be worn by rival Eddie Brock, who will become the supervillain Venom. 
Cast of Characters
- Captain America; Captain Marvel II; Hawkeye; Iron Man II (Jim Rhodes); She-Hulk; Thor; Wasp
- Fantastic Four: Human Torch; Mister Fantastic; Thing
- Solo: Hulk; Spider-Man; Spider-Woman
- X-Men: Colossus; Cyclops; Magneto; Nightcrawler; Professor X; Rogue; Storm; Wolverine; Lockheed the Dragon (Mascot)
- Absorbing Man; Doctor Doom; Doctor Octopus; Enchantress; Kang the Conqueror; Klaw; Lizard; Molecule Man; Titania; Ultron; Volcana; Wrecking Crew
Unlike later crossovers, none of the following issues were bannered or promoted as being part of Secret Wars, but do feature characters leaving/returning from 'Battleworld'. The first issue for each entry denotes departure; the second return.
- Amazing Spider-Man #251 - 252 (Apr. - May 1984)
Note: Spider-Man returns to Earth in a new black costume, with Curt Connors.
- Avengers #242 - 243 (Mar. - Apr. 1984)
Note: The Avengers return without She-Hulk.
- Captain America #292 (Apr. 1984) (Departure)
- Incredible Hulk #294 - 295 (Apr. - May 1984)
- Iron Man #181 - 182 (Apr. - May 1984)
- Fantastic Four #265 (Apr. 1984) (Return)
Note: Mr. Fantastic and Human Torch return with She-Hulk as part of the team.
- Marvel Team-Up #140 - 141 (May 1984)
- The Thing #10 (Apr. 1984) (Departure)
- Uncanny X-Men #180 - 181 (Apr. - May 1984)
A sequel, Secret Wars II, was published in 1985.
Mattel Toy Series
Mattel released two lines of figures (with a series of vehicles and accessories) in 1984 and 1985.
- Marvel Super-Heroes Secret Wars #1 (May 1984)
- Secret Wars #3 (Jul. 1984)
- Secret Wars #6 (Oct. 1984)
- Secret Wars #8 (Dec. 1984)
- Secret Wars #10 (Feb. 1985)
- Secret Wars #12 (Apr. 1985)
- Avengers #243 (May 1984)
- What If...? #114 (vol. 2, Nov. 1998)