Starro the Conqueror has used Rip Hunter to spread his mind-possessing spores across history. To make matters worse, Starro has gathered all of his body-snatched population for the ultimate battle royale, with hero vs. hero and villain vs. villain.
Booster gold wakes up in world controled by Starro, covered in a sea of starro spores, of course he escapes but he then has to contend with starro controled Solomon Grundy, Lady Chronos and Chronos. He gets away with a little help from Skeet, and finds the old Hall of Justice where J'onn J'onzz has hidden a freeze ray, for just such an instance as a Starro attack.
Booster uses it to free Lady Chronos from Starro's control and they escape into the time stream back to when the Starro's attacked. They realize that the only way to stop Starro is to get Rip's time sphere and go back to when Starro first appeared and stop him there.
This plan works but Chronos is able to use the deversion to break into Rip's Lab and steal some valuable information for Rip.