Herbert Wrayson, a bachelor returns to his flat one night to find a young lady rifling his desk. He questions her and finds she thought she was in the apartment of his neighbor, Morris Barnes, who lives above him. While he is on the telephone, she quietly slips out of his flat and heads to Barnes' abode. A few hours later, she is once again at his door – this time looking scared and faint. She asks Wrayson to escort her downstairs as the hallway is unlit. As they emerge, a hansom sits at the doorway with Morris Barnes in it. But, they discover that Barnes has been strangled. Wrayson soon learns that the young lady is the estranged older daughter of a club acquaintance, retired Colonel Fitzmaurice. He also discovers that he has fallen in love with her. The big question however, has he fallen for a murderess? How can he discover the truth? In typical Oppenheim style, this "whodunit" weaves a tangled web and one must wait until the end to discover the surprising truth.