Portrait painter, Charles Honeybath, is intrigued when he is visited by a mysterious Mr Peach and is commissioned to paint an anonymous, aristocratic sitter, known only as `Mr X', whom relatives claim is insane. Under cover of night, Honeybath is taken to the house and asked to stay while he completes his work; but when he returns to his studio, he discovers that the bank next door has been robbed and that he is under suspicion!