Tom doesn't believe that Tubb has seen a Mermaid until he sees a Black Manta Knight chase her down and capture her. Moby and the boys follow them into a mysterious Shimmering Screen where they discover another undersea world. They quickly move into action to try and rescue the helpless Mermaid. The Mermaid warns that they must not attempt a rescue as the Shimmering Screen only appears once every one hundred years. If they don't go back through, they will not be able to return to their world. Tom and Tubb are unfazed by this and vow to rescue her in time for them to all escape. With Scooby's help, Moby and the boys are able to defeat the Black Manta Knight and return to their world just as the Shimmering Screen vanishes.