The inventor of a powerful new explosive, Sir Claud Amory, is somewhat disconcerted when the chemical formula goes missing - the situation worsened by the knowledge that the thief is one of Sir Claud's house-guests. A relative? Or a 'friend'? Whoever the culprit, Sir Claud decides to give them a sporting chance - which is more than someone gives him. As the doors are locked and the lights turned off, rather than return the formula, one of the guests adds something to their host's coffee... Cream? No. Sugar? No - a lethal dose of hyoscine hydrobromide.