Regular guest Francis Marchmont visiting the seaside Hotel Continental, now mostly deserted due to a diphtheria epidemic, meets and befriends a mysterious woman, "Mrs. Lucie Fairbanks," only to discover she and another woman are rivals, both common-law claimants to the fortune of his hated enemy, the late copper magnate Thomas Cadwallader Bennet, leading to a night of intrigue and mistaken identities in the empty hotel.