A shot rings out in a darkened apartment; a woman screams and flees, tricking architect Jimmy McMillan into giving her a ride. McMillan returns and finds a body; but the police find a different body! He and the woman in question (Mary Rawlins, niece of the late owner), both suspects, become allies; among light romantic banter they hunt the real killer behind the seven doors of a blind alley, housing eccentric suspects and a dark, intricate, spooky cellar...