Green Lantern (John Stewart) and Atom (Ray Palmer) are on perimeter patrol on the moon. Something has landed on the moon and they’re checking it out. They find the object and Atom shrinks down and enters it. Then he forgets how to enlarge himself. On the surface of the moon, John forgets how to use his power ring. Flash runs out to the moon, grabs John, and brings him back inside.
He continues running though, as he has forgotten how to stop.
Ray reports that the machine he is in is some sort of alien device, devised to retrieve and store specific types of memories, accounting for the recent memory losses. At the center of the device, Ray finds a black hole no bigger than a speck of dust that is contained by a magnetic field.
Batman decides to fill in some of the other Leaguers about what is happening when he forgets the English language. The device turns out to be a synthetic life form that chooses the memories it takes. It calls itself Mnemon.
Wonder Woman and Firestorm go the lunar base and Wonder Woman stops Flash with her lasso. She asks Green Lantern what’s going on and he tells her he’s not sure because he doesn’t remember much, but he thinks it has to do with the object on the table.
Firestorm shoots a stream of fire at the object, but Superman arrives and blocks the flame from hitting the object. Wonder Woman and Firestorm forget who Superman is and begin fighting him, thinking him a villain.
The Atom escapes Mnemon by shrinking and passing between the molecules. As soon as Atom is free, Superman destroys the device with his heat vision.
Now the black hole is free of its device, Superman grabs it and tries to contain it in his fist. John and Superman fly the black hole to a desolate stretch of space and hurl it away.