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 household.
Seeking a swift remedy, Sir Claud locks the doors and switches off the lights to allow the thief to replace the formula, no questions asked. But the darkness brings death, and Hercule Poirot is left to unravel a tangle of family feuds, old flames and suspicious guests to find the killer and prevent a catastrophe