At Alice's birthday party somebody, or some animal, has ruined the birthday cake. Facing accusations from Alice's brother and his cat, Nelson (Martha's nemesis), Martha proclaims her innocence in spite of overwhelming cake crumb evidence. Finally clearing her name, Martha moves on to apply her detective skills to uncover a plot to rob a local jewelry store. But it turns out that the main suspects are police detectives on a stakeout. Has Martha foiled the stakeout plan for good, or can she rally her canine friends to help her catch the real criminals?