Kirk Langstrom is in a park, waiting for his wife, or the woman who pretended to be his wife in order to steal the Man-Bat formula. But where normal men may expect their wives to just walk up to them, Kirk is expecting his to show up as She-Bat, hunting prey. Sure enough, she does, and right after Francine snatches up someone, Kirk doses himself with Man-Bat serum and gives chase, eventually catching up to, and defeating his wife, who reverts back to herself. Francine realizes that Kirk must have mixed their two formulas together to beat her, and laughs at him, for not understanding what he's done to himself. Before Kirk can do anything, the police show up, and arrest Francine.
With Batman gone, Kirk develops somewhat of a hero complex, and works tirelessly to perfect his formula, in order to try and keep peace in the streets. He keeps a daily diary of his changes and advancements, but he starts to lose himself in the way his wife predicted.
Where Kirk starts off more virtuous, saving innocent families from thugs, he starts getting paranoid and attacks cops who he believes to be dirty, and jealous of his heroics. His attacks get more and more violent to the point where he's killing some of the people he attacks. Meanwhile, while he's not in his Man-Bat form, he becomes more and more obsessed with perfecting his formula, up until the point where he completely loses himself, ending up doing exactly what his wife Francine did, and hunting innocent civilians to feed.