Batman and Robin are invited to a top-secret demonstration hosted by Professor Hugo Strange and his assistant Dr. Harleen Quinzel that may eventually change the future of Gotham forever. Along the way, Batman takes a stop at Gotham State Penitentiary to visit Catwoman (whose suicide attempt from the first movie somehow failed) to give her a book on poetry by Elizabeth Barrett Browning. This is something to keep her occupied for the next 36 months of her life until she is let out of prison for good. At the laboratory, they meet district attorney and Bruce Wayne’s ally in the war against crime Harvey Dent who is famous for having stopped a coin counterfeiting gang while keeping a two-faced quarter as a souvenir from the case.
Hugo Strange believes that good and evil are all but one-sided and has created an “Evil Extractor” to which the volunteering criminals Joker, Penguin, Riddler, Egghead, and Mr. Freeze, will be purified of their corruption. Batman bluffs it off, saying that there are no easy fixes to the straight and narrow path, but Strange conducts his experiment anyway, despite the fact that it will make the jobs of Batman, Robin and Harvey obsolete. All goes well until the Joker entices his fellow villains to overload the machine. Noticing this, Professor Strange orders Dr. Quinzel to shut it down…but is too late and Harvey (as opposed to acid from the comic book mythos) is horribly scarred by the explosion of the containment vat, with Batman being successful in saving the right side of his face.
Harvey, now re-christened Two-Face, goes on a crime spree as seen in the opening credits, all of which his attempts are foiled by the Dynamic Duo. He is eventually whisked to a hospital where his face has been given reconstructive surgery. His sanity has apparently been restored as well.