In the absence of her husband, who is away on a business trip, Alleyn's wife, the talented artist Troy, goes on a pleasure cruise along the Thames on a small steamer. The atmosphere on the ship is cozy, despite the motley, varied company of passengers. But something seems subtly suspicious to Troy, something makes her tensely wait for events. And her expectations are justified when someone brutally kills an annoying, funny, but good-natured old maid. Troy decides to start his own investigation...