Inventor Sir Claude Amory feels a bitter taste in the mouth, when the new formula for explosive material stolen by someone in the household.

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.