An alien race called The Dominators discovers that Earth's superhumans get their powers from a 'meta-gene' and decide to capture them all to be used as living weapons with which to conquer the universe. Realizing that they cannot beat the heroes by themselves, they convince several other races from around The DC Universe to aid them under the pretense that the "metahumans" were too dangerous to be allowed to exist (and taking advantage of the fact that the Green Lantern Corps had, at the time, been reduced to only a handful.)
The combined might of the alien armada seemed invincible but thanks to a Heel–Face Turn by one of its own member races, it was routed. But a second threat happened when a Dominator scientist detonated a "meta-gene bomb" whose radiation rendered most of Earth's metahumans helpless. Another switching of sides—by, ironically, that same scientist—helped save them.
The series L.E.G.I.O.N. (about a mercenary group that protected the galaxy in the Corps' absence) was spun off from the crossover. It also launched the series of Justice League Europe and the Daxamite superhero Valor (also known as Mon-El). (A team called The Blasters was introduced by the crossover and given a one-shot special, but never had its own series.)
- Action Bomb: Lex Luthor contributed by making robotic replicas of heroes that would get captured, and then explode while being studied.
- Aliens in Cardiff: The alien armada begins its attack by... taking over Australia.
- Bald of Evil: Thanagarian men are bald fascists.
- Captain Ersatz:
- Category Traitor: The Thanagarians joined mostly to capture Katar and Shayera Hol.
- Easily Thwarted Alien Invasion: Earlier attempts by various races. This one came closer to succeeding however.
- Eldritch Abomination: The Gil'Disphan were the Invasion's submariner force, and looked like tumors floating in bubbles.
- Evil Is Petty: Part of what stopped the invasion was the various races sniping at each other, helped along by Deadman possessing some.
- Fantastic Caste System: The Dominators, to their disadvantage. The scientist who invented the gene bomb was only told to study the meta gene because of his caste status. His subsequent torture for going outside what his caste was permitted to do was what allowed the heroes to convince him to make a cure.
- Fantastic Racism: Part of the reason for the alliance was... that aliens don't like how we look. The Daxamites and Thanagarians resembled Earth's humans, but they had no genetic variance eg. they all look alike.
- Heroic Sacrifice: One of the Daxamite observers for the alien alliance, who died of lead poisoning so he could reach a transmitter to tell his homeworld to throw in on Earth's side. (At the time, the Daxamite was the father of Lar Gand, a.k.a. Mon-El; this has probably been retconned.)
- Human Aliens: The Daxamites and Thanagarians.
- Irony: Several things.
- The invaders had no idea that the superhumans were going to be used against them once captured.
- The Daxamites helping route the invasion.
- A Dominator helping save Earth's heroes (after almost killing them.)
- The Thanagarians' original reason for joining the Invasion was to capture Hawkman and Hawkwoman, rogue Thanagarians who had defected to Earth. The popularity of the Hawkworld comic would lead to these characters being rebooted, and retconned the previous Hawkman and Hawkwoman members of the Justice League being advance agents of the Invasion who remained loyal. Hawkworld came out literally months after the conclusion to Invasion!, essentially making the Thanagarians' involvement immediately incoherent.
- Meta Origin: The meta-gene, present in some but not all humans. It grants superpowers when exposed to dangerous forces. Note that not all superheroes got their powers from it, just the majority.
- The "Blasters" team were the survivors of a group of 100 random humans kidnapped to demonstrate the theory. The Dominator running the experiment expected that if they were all attacked by a firing-squad barrage, one of them would spontaneously develop powers that would allow him to survive. Instead, seven people did, including (unknown to them) former Justice League sidekick Snapper Carr.
- The Daxamite gain powers on Earth similar to Superman's, apparently being distant cousins of Kryptonians. However they were also vulnerable to lead-based pollution, and only survived because Earth's heroes saved them.
- Some people actually gained powers (or had them altered) as a result of the meta-bomb, most notably Justice League manager Maxwell Lord. Several of the new members of Doom Patrol introduced during the Grant Morrison run were also said to have got their powers from the meta-bomb.
- Metamorpho actually came back from the dead because of it. Notable not only in that it implied his powers were genetic rather than mystic, but also that the gene bomb could work on the dead.
- Names to Run Away from Really Fast: The Dominators. Not that most of their allies realized it.
- Parody: At the time, Marvel's X-Men were situated in Australia. So they had an issue where they stopped a thinly-disguised version of DC's invasion.
- Red Skies Crossover: Some comics had no direct ties to the event beyond a single black and white panel with reversed coloration, signifying the moment of the gene bomb's detonation.
- Rubber-Forehead Aliens: Most of the alien races that weren't the Daxamites, Thangarians, or Gil'Dishpan.
- The headline for the Daily Planet in response to the United Nations rejecting the Alien Alliance's demand to surrender their superheroes reads, "EARTH TO INVADERS: DROP DEAD!", a reference to the October 29, 1975 issue of the New York Daily News, whose headline reads, "FORD TO CITY: DROP DEAD".
- The cover to the second issue is a reference to the US Marines hoisting up the US flag after the battle of Iwo Jima.
- Space Police: The Green Lantern Corps, the L.E.G.I.O.N.
- Weaksauce Weakness: Lead, for the Daxamites.
- You Have Failed Me: The Dominator Leaders had not told their scientist what the real purpose of their mission was. So they rewarded his invention with torture, which led to his Heel–Face Turn.
- Your Princess Is in Another Castle: The invading alliance seems to be defeated by the second issue... oh, wait, gene bomb.