An omnipotent Cosmic Entity by the name of The Beyonder, fascinated by the presence of superheroes on Earth and their potential, chooses groups of both heroes and supervillains and teleports them against their will to "Battleworld", a planet created by him in a distant galaxy, stocked with alien weapons, technology and mismatched parts from different worlds. He tells them to kill each other, and, to the victor, his greatest desire will be granted. His purpose is to understand the (to him) alien concept of "desire".
The heroes include the Fantastic Four (minus Sue Storm, who was pregnant at the time), The Mighty Thor, the Jim Rhodes version of Iron Man, the Monica Rambeau version of Captain Marvel, She Hulk, Spider-Man, The Incredible Hulk, Hawkeye, Captain America (comics) and The Wasp, with the second Spider Woman (Julia Carpenter) showing up several issues in (in her first appearance). Members of the X-Men (Professor X, Cyclops, Storm, Colossus, Wolverine, Nightcrawler, Rogue, and Lockheed The Dragon) split off from the main heroic team and act as a separate faction for much of the book. Magneto, buffing up his Anti-Villain credentials, also does his own thing for awhile before joining with his fellow mutants.
Villains include Doctor Doom, Doctor Octopus, Klaw, Ultron, The Lizard, The Wrecking Crew, The Enchantress, Kang The Conqueror, Molecule Man, The Absorbing Man, and new villains Titania and Volcana. Galactus The Planet Eater is also there acting as his own non-allied entity.
So, as you might guess, new friends are made, old alliances are tested, complicated intrigues are formulated, and in the end, everyone gets home without anyone dying, though Spidey gets a shiny new black costume inspired by Spider Woman's (which would later become the Venom symbiote), She Hulk replaces The Thing in the Fantastic Four (who stays behind after he finds he can revert to human form on the Battleworld), Volcana hooks up with Molecule Man, and Colossus gets Strangled by the Red String to an alien healer, causing him to break up with Kitty Pryde. The toy line sold poorly, but the comics sold well, and the whole thing was continued the next year in Secret Wars II in which the Beyonder came to Earth and tried becoming human. This series was seen by some as a Self-Insert Fic by Jim Shooter (writer of both series and current Editor in Chief of Marvel at the time.)
The Beyonder got retconned into a delusional lesser cosmic being, not really omnipotent (still more powerful than Earth's heroes, however) in a Fantastic Four story years later, in what was apparently a Take That at Shooter. Steve Englehart, FF writer at the time, reported that this was editorial interference.
There was a miniseries in the 2000s titled "Secret War" but it had nothing to do with the first two. Instead, it was about superspy Nick Fury getting some heroes to help him track the source of supervillain technology (which turned out to be Doctor Doom), and ended with Fury being removed as leader of SHIELD for acting without permission.
There was also a miniseries entitled "Beyond!" in which the events of the first series seemed to be playing all over again, with a reduced cast of heroes and villains. It was finally revealed not to be the Beyonder, but The Stranger pretending for "research purposes".
A pared down version of the first Secret Wars was adapted for Spider-Man: The Animated Series, reducing the number of characters involved and making Spider-Man the leader of the forces of good rather than Captain America. It was well-received and is considered one of the greatest episode arcs of the show.
- Anti-Villain/ Anti-Hero (Type IV): While this might not be Magneto's most heroic period (he's very conniving in the early issues), it's certainly his most publicly heroic. To wit:
- When the heroes and villains are first gathered by the Beyonder, Magneto is sorted with the heroes. The Beyonder classified them due to their desires, and Magneto's goal of mutant supremacy was more noble than the other villains' desires of personal power and wealth.
- During the last few issues, he accepts Captain America's leadership and fights alongside the heroes without hesitation.
- Towards the end, when it looks like the world's tearing itself apart, Cap runs into the heroes' HQ to free captured villains from their cells, so they're not crushed to death. He learns this was also Magneto's first impulse.
- Awakening the Sleeping Giant: Molecule Man. Up till this point, he made very limited application of his powers and believed he was limited to inorganic matter and was very meek and submissive. Over the course of Secret Wars I and II, he first learns that he is capable of affecting all matter on any scale and slowly overcomes his meekness to the point that he goes toe to toe with the Beyonder in the second book having accepted that he's the second most powerful being in the universe.
- It should be pointed out that early in the series, before he gets starts getting more powerful and confident, he is able to casually drop a mountain on the heroes without so much as breaking a sweat.
- Followed not too long after that by ripping an even larger chunk of the planet's crust (over 2 billion tons of matter) and sweeping it up into almost the vacuum of space with all the heroes on it because he was angry at Doom who had just gained the Beyonder's powers and wanted to get everyone else out of his way and he did it just as casually as waving a hand. That being just before Doom uses the Beyonder's powers to remove Molecule Man's self-imposed limitations.
- Battle Royale With Cheese
- Be Careful What You Wish For: Although the characters don't realize it, Battleworld was granting their wishes all along. This is why both Colossus and The Human Torch find "the perfect girlfriend" (Zsaji) and The Thing can change back to human.
- Actually the Thing was revealed prior to that that he had already gained that ability but his belief that his GF could only love him as the Thing had been the reason Reed's attempts at cures had been failing in recent years. Reed never told him (leading to a What The Hell, Hero? moment for Reed) and only the idea that they likely would never return to Earth broke down those limitations.
- Christmas Rushed: Marvel's corporate offices ordered production on this series sped up so they could get it to stores before DC's Crisis on Infinite Earths. It shows in some of the dialogue and the ham-handed (in-story) way in which the various heroes and villains were shoe-horned into it.
- C-List Fodder: Surprisingly averted. The Wasp gets shot through the heart at one point, and Doom offs a couple of minor villains out of pique, but in the end they're Only Mostly Dead.
- Continuity Snarl: Doctor Doom was dead at the time of this series, but was too big a baddie to be left out. The Fantastic Four comic spent much of the next year unraveling how Doom could be present for the Wars while dead.
- Convection, Schmonvection: When the Human Torch uses his "nova flame" to take out Ultron, it burns so hot that it melts clean through the surrounding solid-metal walls and floor. When the smoke clears, Captain America, who was standing a few yards away and protected only by huddling his upper body behind his shield, is perfectly unharmed. Apparently, his body's melting point is a lot higher than that of whatever alien metal was used to build Doombase.
- Well, obviously. Mundane fire, no matter how hot, will never burn bright enough to melt AMERICA.
- Crisis Crossover: Though the whole story was presented in its own mini-series.
- Curb Stomp Battle: Spider-Man using his vastly superior agility, reaction-time, and combat experience to take out Titania. She not only never landed a punch on him, but he took his time while delivering her beat down, all the while delivering a classic "The Reason You Suck" Speech. It made her fearful of Spider-Man for years, despite the fact that she is much, much stronger and Nigh Invulnerable.
- Earlier, She-Hulk breaks into Doombase alone, to take revenge for the Wasp (supposed dead at the time). She briefly holds her own against the Wrecking Crew, but the addition of Titania, Doc Ock, and the Absorbing Man to the fight results in them beating her nearly to death. The pummeling she takes verges on Nightmare Fuel.
- Even earlier Spider-Man comes upon the X-men plotting to join Magneto, he prepares to tail on them and easily beats down Wolverine, Rogue, Cyclops, Storm, Nightcrawler and Colossus before getting mindwiped by Xavier.
- Disaster Democracy
- Double Standard: When gathering bad guys, the Beyonder was apparently only able to find one A-list villainess, the Enchantress. (Marvel had a lack, at the time.) Doctor Doom later rectifies this by empowering two civilians into Titania and Volcana.
- Executive Meddling: Colossus' falling for Zsaji was the result of writer Jim Shooter disliking the awkwardness of pairing the 19 year old with 14 year old Kitty Pryde.
- Evil Costume Switch: Spidey gets a new black costume, later revealed to be alive.
- Although, at first, the "Black Costume is Evil/Alive" wasn't introduced at the time, and that the black costume was just introduced as a new costume for Spidey.
- Evil Tower of Ominousness: Doom builds himself a 200-mile high tower made of golden stone as his temporary quarters, and invites the heroes to meet with him.
- Fridge Logic: OK, so you're an alien entity that has never known the concept of desire. So you decide to explore it by... having a bunch of heroes and villains fight each other? (To his credit, the Beyonder was almost as innocent as a baby at the time.)
- It was implied that the Beyonder had spent at least some time spying on the normal universe - since the source of the hole between universes was the accident that created the Molecule Man, 20 years real-time (and at least a few years comic-time) passed between the formation of the hole and when he finally gathered up the heroes - and chose the heroes and villains because most of them seemed far more passionate than the average person. The upshot was essentially that they had different TYPES of passions (with the heroes being more selfless), and that by getting them to fight, he could learn just how far people were willing to go to satisfy their desires.
- Genre Savvy: Doom seems to immediately realize the deck is stacked against him in the good vs. evil battle, and spends most of his time scheming to steal the Beyonder's power. Which he does, briefly.
- Hold Your Hippogriffs: Colossus once exclaims "Lenin's beard!"
- Hollywood Pudgy: Volcana isn't as fat to get all those insults from Enchantress.
- Of course the Enchantress is supernaturally beautiful, legendarily vain, and and doesn't have much use for mortals to begin with; it's entirely possible she was just zeroing in on something Volcana was self-conscious about and exaggerating it to make her feel bad. Of course, later artists tend to draw her much chubbier, so either she gained weight after the mini-series or she was supposed to be larger than the art depicted her.
- I Broke a Nail: Wasp
- Load-Bearing Hero: The Hulk holding up a mountain range.
- Oddly, the toy line bore almost no resemblance to the comic. There were only four heroes and four villains (including Kang, who dies early in the series, and Magneto, who's a hero in the comics) in the first set of toys, and each had a shield supposedly used to send secret messages (only Captain America had a shield in the comics, and it was his usual, non-message-sending one). The second set of toys added characters who weren't even in the comic (Falcon, Daredevil, Baron Zemo, etc.).
- Funnily enough, there's now a current series of toys involving reprints of Secret Wars and Secret Wars II, which has fewer "continuity" problems than the original toy line.
- Not That Kind of Doctor: A fellow supervillain asks Dr. Octopus to help an injured colleague because he's a "Doctor," he points out that he has a doctorate in nuclear physics.
- Original Generation: The Beyonder
- Patchwork World
- Pivotal Wakeup: Galactus.
- Plot Hole: A retroactive one at least. When Spider-Man gets his new black symbiote costume he celebrates by playfully squirting the Human Torch in the face with the costume's built-in web shooters, and the Human Torch comments that this new stuff is "even harder to burn off than your old webbing!" However, later comics established that A) the "webbing" created by the symbiotes are actually cast off pieces of the symbiotes themselves made to look like webs, and B) symbiotes are weak against fire. Given that, logically Spidey's new webbing should be easier for Torch to burn off, not harder.
- Power Parasite: Doctor Doom steals Beyonders' ability here.
- "The Reason You Suck" Speech: Spider Man verbally eviscerates Titania. It's brutal to read but well deserved, especially since Titania had just delivered an awful beat down of She-Hulk.
Spider-Man: "You ought to be happy, cuddles! You aspired to be a bully, and, man, you're a classic! You talk tough and nasty when you've got the upper hand--But when you're losing--well, that's when the whining little wimp-ette inside comes spilling out!"
- Recursive Adaptation: It was a comic based on the toys. Now, there are the toys of the comic of the toys. Not sure if the Animated Adaptation of the comic of the toys count, though.
- Screw This, I'm Outta Here: Molecule Man, once his mental blocks are removed by Doom, decides to just take any villain who wants to back to Earth.
- The Eighties: Oh YES. Most noticeable when Johnny Storm tries to woo Zsaji by singing lyrics to Michael Jackson and Culture Club songs.
- Token Evil Teammate: Magneto, who was thrown in with the heroes.
- Token Minority Couple: Averted. Rhodes!Iron Man tried flirting with Rambeau!Captain Marvel, but she could tell there was someone different under the armor, and wasn't quite as impressed with the new guy.
- Totally Radical: Some of the dialogue. She-Hulk actually says "TO THE MAX!" at one point.
- Tournament Arc
- Tweener: Magneto throughout, and Galactus to a certain extent. The Lizard was mostly there by accident, and doesn't seem to have any allegiance to anyone, save Wasp, who bandaged up his injuries.
- Galactus is simply above it all. From his perspective, he's been transported by a superior alien being to a new world alongside some rodents, insects, and a couple of chimps (Reed and Doom) so he spends most of the series ignoring them and trying to engage the alien.
- What If: An issue told the story of what would have happened had the heroes never gotten home. Notably:
- Rogue permanently was consumed by the personality she'd absorbed from Carol Danvers.
- Spidey and the Symbiote are now one, to the point that when the Symbiote is hit by a sonic attack, there's nothing left of Peter underneath but his skeleton.
- Professor Xavier now wears an upgraded Iron Man suit to protect his health and allow him to walk.
- Several, including Magneto, James Rhodes, and Reed Richards, have died in the twenty-five year interim.
- The Hulk disappears, becoming something of a bogeyman.
- There are kids, who have powers based on a combination of both parents'. The good kids are Crusader (Cap and Rogue/Carol), Bravado (Thor and Enchantress), Mustang (Hawkeye and She-Hulk), Firefly (Human Torch and Wasp), and Torrent (Wolverine and Storm). The bad kids are Malefactor (Dr. Doom and Enchantress), Chokehold (Absorbing Man and Titania), Moleculon (Molecule Man and Volcana), Gator (Lizard and...someone), and Raze (Wrecker and unidentified woman).