An urbane man discovers an abandoned cab with a sleeping horse and, following clues found inside, becomes entangled in a mystery built around a photograph, a ciphered message, and a discarded handkerchief. Investigation reveals a glamorous woman who assumes multiple identities and operates within intelligence networks; the plot advances through decoys, intercepted letters, taxi pursuits, and confrontations. Loyalties shift as secrets are exposed, culminating in a tense capture and the resolution of tangled deceptions.