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!