Five people, all united by the fact that they are on a train bound for Monte Carlo, will find themselves involved in the murder of an elderly millionaire of Dutch origin. The spouses Remo and Marina, hairdressers, are attracted by the mirage of luck in gambling, with which they plan to start their own business; Commendatore Alberto Franzetti, after a failed attempt to "detoxify himself" from the game demon by Padre Pio of Pietrelcina, is back in Monaco, where his wife is waiting for him; the burgher couple Quirino and Giovanna are busy bringing a dog back to its Dutch millionaire owner for a lucrative reward. The six protagonists, for different reasons, are involved in the investigation into the murderer of the old millionaire who had lost her dog and, all wary of the police, they carry out clumsy attempts to prove their strangeness to the fact, ending but only by increasingly compromising their position; but thanks to the investigations of the police commissioner they will be acquitted, while the real culprits (who had ingeniously provided themselves with alibis) will be unmasked.