Tom, Tubb and Scooby are water skiing behind their friend Moby when they notice what looks like a space capsule about to splashdown in the ocean. Manta Menace is waiting for the capsule and it's contents, the secret maps of the Mars diamond mines. The boys see the capsule sink and have Moby take them in so they can investigate. Manta Menace spots them and begins firing torpedoes and then trying for a getaway in his Sinister Sea Saucer. Moby and they boys manage to avoid the torpedoes and continue their pursuit of the evil Manta Menace. Manta Menace sends several creatures after Moby and the boys but due to quick actions by Moby they avoid them all and are able to stop Manta Menace before he can deliver his maps to the enemy agents.