When Ariel reads a thrilling biography of the famous mermaid spy Mata Herring, she starts to do some snooping around herself. Suspecting that her substitute teacher, Mr. Kipper, is up to no good, she convinces her best friend, Coral, that Mr. Kipper has kidnapped their real teacher, Mr. Chubb! Unfortunately, nobody believes Ariel's outrageous accusations, and King Triton demands that she stop her spying routine. But in the tradition of the great Mata Herring, Ariel knows she must continue her quest and help save her teacher - even if it means going against her father's wishes!