Mister Freeze attacked a park in Gotham with his freeze-cannon, freezing several families. He was unimpressed when Robin showed up to stop him, apparently alone. Robin managed to crack his helmet, allowing Batman to smash it and take Freeze back to Arkham Asylum.
Later in September, Freeze successfully petitioned the courts to be declared legally sane and was transferred to Belle Reve Penitentiary on September 16 along with Icicle Jr., Miss Martian, and Superboy (the latter two disguised as the Terror Twins). Once there, he and Icicle Sr. put a plan into action to break out every super criminal in Belle Reve. First, he had Professor Ivo distribute thermal jumpsuits to the inmates to protect them from the cold, and then he assembled freeze weapons using components smuggled into the prison with the jumpsuits. He staged a coup on "capo" Icicle Sr. to get sent to the Warden's office where he froze the guard and deactivated the Inhibitor collars. The last part of the plan involved all ice-themed villains to make the walls brittle, so villains with super strength could break them.
The plan was thwarted when Superboy tricked Icicle Jr. into helping him reactivate the collars. When Mister Freeze confronted them about it, Superboy turned Icicle Jr. against Freeze and smashed his dome open, forcing him to freeze himself to stay alive.
After five mysterious ice fortresses covered the United States with a blanket of snow, Batman and Flash suspected Mister Freeze and the other ice-villains were involved. Hugo Strange told them they had been, and still were, in their cells.However, Mister Freeze was in fact responsible for powering one of the fortresses.
This is the seventh animated incarnation of Mister Freeze. Previously to Young Justice, he appeared in The Batman/Superman Hour, The New Adventures of Batman, the DC Animated Universe (Batman: The Animated Series, the direct-to-video film, Batman & Mr. Freeze: SubZero, The New Batman Adventures, and Batman Beyond), The Batman, and the direct-to-video animated film, Superman/Batman: Public Enemies, and Batman: The Brave and the Bold (both as Mr. Zero and Mr. Freeze).