At the Number One Belgian Detective Agency, Pierre has left young lovers Marcel and Monique in charge. But after four months, they still haven’t worked out who killed Art Garfunkel, Director of the Museum of Modern Art. In the absence of any evidence, the police arrest Brigitte, Marcel’s sister. After all, in Belgian law, unless there is any evidence to the contrary, it’s always the cleaner who dunnit. Marcel must save her from the guilllotine, but with Monique admitting to the murder, he must choose between his sister and his love. But what of Pierre?