An apartment handyman is unusually attached to a pair of boilers he names "Mavis" and "Ethel."
Rodney Playfair is persuaded, by a promise to meet his gambling debts, to impersonate a manservant named Chapman at his fiancée's house...
When she learns that the Nazis have confiscated Leonardo da Vinci's masterpiece Mona Lisa, art-loving South American wanna-be spy Tamara (Jane Carr) journeys to Lisbon to snoop for the Germans in return for the iconic painting. But bumbling Nazi agents, Allied counterspies and multiple copies of the artwork soon confound her attempts at espionage.
