When a mass outbreak of super-powered individuals threatens the world, the JLA determines that latent metahuman abilities are being triggered by an alien contagion, one that's spreading throughout Earth as it transforms its hosts into monstrous engines of destruction. Racing to contain the infection, the World's Greatest Super Heroes link the parasitic plague to an alien invasion from the team's earliest days and make a horrifying discovery: the JLA itself is responsible for the imminent disaster they now face.