Fearing for the security of his revolutionary new formula for a powerful explosive, Sir Claud Amory requests the assistance of Hercule Poirot in transferring it safely to the Ministry of Defence. But when the formula disappears from its safe, Amory must change his plans. Locking his houseguests in the library, he informs them that the thief has precisely one minute of darkness in which to return the formula anonymously - or face the great detective even now on his way. The lights go off; by the time they come on again, Amory is dead in his arm-chair ...