Michele and Gennaro are caretakers at the detective agency 'Nothing Escapes'. When their boss is forced to leave the office to follow an investigation, the two decide to temporarily take his place and pose as investigators in front of two elderly sisters. These women have come to the agency to request an investigation into the morality of their only nephew's wife, an American doctor originally from Boston.
The doctor has been called in to replace the famous Professor Vagoni and take care of his patients, among whom is the Marquis De Vitti, a nobleman with a seductive nature who has been wounded in the backside by a peasant who tried to seduce the Marquis' wife. The reason why the marquis would like to be treated by a doctor instead of a lady doctor is due to his shyness in confessing this openly.
The two investigators mistakenly believe the marquis to be the doctor's lover and try to provide evidence to prove the doctor's alleged betrayal of the two sisters.
To confirm their assumptions, Michele and Gennaro pretend to be a doctor and a patient respectively, claiming to be waiting for an appendicitis operation. So they are admitted to the Villa Valeria clinic, where the doctor works, and where Marquis De Vitti is also admitted. In the end, their clumsiness is discovered, and the story ends happily.