The inventor Sir Claud Amory is left with a bitter taste in his mouth when the formula for his powerful new explosive is stolen-by someone staying in his own house. In order to quickly remedy the situation, Sir Claude locks the door and turns off the light, giving the thief a chance to return the formula without being detected. But darkness brings death and Hercule Poirot has to untangle family strife, love and suspicious visitors tangle in order to clarify the murderer and prevent disaster.
