Members of the Justice League are falling one by one to a strange mental illness, and it's contagious.
The Jest-Master nears Thanagar. The Hawkman, and the Atom, are also en route to Thanagar. THe Atom's girlfriend, Jean Loring, is hopelessly insane. Loring is being brought to Thanagar for treatment. Thanagarian defenses fire on the Hawkman's ship. After identifying himself, the Hawkman docks at the outpost. Thanagarian law enforcement officers, clearly deranged, attack the Hawkman, and the Atom. The strange madness afflicting the outpost proves contagious. Soon, the Atom, and the Hawkman, are locked in mortal combat with each other.
On board the Hawkman's vessel, Norch Lor, a Thanagarian criminal escapes his restraints. Lor is the only man aware of the coming of the Jest-Master. Only Lor knows of the Jest-Master's ruinous effect on planetary populations. In a misguided attempt to save Thanagar, Lor had stolen the Gehnna Box, an ancient soul stealing artifact. Lor had intended to preserve the souls of Thanagar within the Gehenna Box, saving them from the Jest-Master's destructive influence. While testing the Gehenna Box on Earth, Lor was apprehended. Lor sends out a distress signal from the Hawkman's vessel.
The signal is received by the Guardians of the Universe, who forward it to the Green Lantern of Earth. In turn, the Green Lantern passes the distress signal on to the Justice League of America's satellite headquarters, where it is received by the Black Canary. The Black Canary summons the Justice League of America, who depart for the Thanagarian outpost. The team arrives just in time to stop the Hawkman from killing the Atom. Enraged, the Hawkman throws himself against Superman, knocking himself out. On board the Hawkman's vessel, Lor attacks, and is subdued by, the Batman.
Loring, her sanity seemingly restored, warns the Justice League of America to leave the outpost. Lor reveals his knowledge of the Jest-Master. Outside of the Jest-Master's influence, the Hawkman and the Atom regain their sanity. Strangely, Loring loses hers again. The Flash volunteers to enter the Jest-Master's sphere of influence, to learn what the team is up against. The Flash barely survives the experience with his sanity intact. Realizing that Loring's sanity seems to be restored by exposure to the Jest-Master's sphere of influence, the Justice League of America drafts Loring to lead their assault.
The Atom protests, but relents upon realizing the team has no other choice. The Justice League of America is completely incapacitated by the journey through the Jest-Master's sphere of influence. Loring, however, makes it to the Jest-Master's inner sanctum unscathed. The Atom confronts the Jest-Master. Decades ago, the Jest-Master's civilization was wiped out by a plague that drove it's populace insane. Only the Jest-Master, and a few others, proved immune to the plague. Now the Jest-Master travels the universe, spreading the plague to other worlds, in the hopes of finding the one world that can cure it. On that world, the Jest-Master will begin a new life.
Rapidly changing his height, the Atom attempts to unnerve the Jest-Master. Fearing that he has finally succumbed to the plague, the Jest-Master lashes out at the Atom. The Jest-Master commands his minions to execute the Atom, but the Flash disarms them. Loring has revived the Justice League of America. The team quickly defeats the Jest-Master, and his minions. Loring's long exposure to the Jest-Master's sphere of influence has completely cured her insanity.