From the Animeraider:

Kai Masayoshi and Fuwa Masami are smart-aleck detectives. They try to act like the cops from Miami Vice but they aren’t quite cool enough to pull it off. After one arrest that goes wrong their captain adds a third to their group, motorcycle cop Morinaga Mai, a pretty young cop who kicks Fuwa in the groin when he tries to come on to her. They get along just fine after that.

But unfortunately, the drink they take together to toast their new partnership is actually a youth serum created by a mad scientist on retainer with the police department – and all three have been turned into children. But they’re still detectives, and they’re going to keep on doing their jobs until they can be turned back into their normal ages again!