Murder At Midnight

Predictable murder mystery in which a roomful of people are murdered one by one after Thomson accidentally kills his secretary with a gun during a game of charades (somebody slipped real bullets into the chamber). Thomson goes next, and then another and another, until it is revealed that Thomson's wife's lover, Hamilton, a noted criminologist, was the killer all along. He commits suicide (with a phone that sends a sharp spike into the brain through the ear--his own invention) which leaves the police nothing to do but clean up.