Jack Costello, nicknamed Extralarge, is a very large ex-cop who works as a private investigator in Miami.

Jack Costello (Bud Spencer) is a retired cop and now private detective who lives in Miami, who handles a myriad of cases for unusual clients. At his side are old police buddy Sam Bosley (Lou Bedford) and cartoonist-turned-amateur sleuth Jean Philippe Dumas (Philip Michael Thomas), who is using Costello as an influence and has nicknamed the burly detective "Extralarge." In the second series, Costello faces other dangerous cases, again with Bosley and Archibald (Michael Winslow), the son of an old friend, who Costello amusingly nicknames "Dumas."