Mon-El, immune to his serum, is back in the Phantom Zone! But can the Legion find a cure?

Mon-El is dealing with the reality of Brainiac 5's serum, the serum that temporarily cures his lead poisoning, failing to work anymore. He's gained an immunity to the serum and he may die or be forced to enter the Phantom Zone again. Unfortunately, it's crunch time and Mon-El is forced into the Phantom Zone for his own health, but he resents being there. Element Lad and Brainiac 5 realize that they need to find a cure immediately or they will lose Mon-El forever. While Brainiac 5 rushes for a lasting antidote, Phantom Girl and Tellus enter the Phantom Zone to calm Mon-El down. Meanwhile, a group of Legionnaires travel to the past, to get the aid of Superboy in handling Mon-El. Taking some of Superboy's blood, Brainiac 5 is able to come up with a permanent cure for Mon-El's predicament.