Father-and-son lion tamers, Rabbas and Léo share a mistress - Rosita, the " lovely bitch " of the film's title. She is as wild as their lionesses and, in a single night the father is injured and the son attempts suicide. Only when Rosita leaves the circus can things get back to normal.

With the carnival in full swing, the circus ringmaster is waiting for the new lion tamer, Margante. A young girl introduces herself under this name and is hired on a trial basis. The real Margante introduces herself in turn, but soon runs away, terrified by the story the young impostor tells her of an imaginary accident that occurred in the lion's cage.