Ships are being damaged by an unknown agent hidden at a depth impossible for the Navy to reach, where the Cetacean finds an automated robot, the Mud Worm. It is programmed to crawl along the sea bottom collecting mineral samples, but was damaged, and now resists all efforts to approach it with deadly force. Mark manages to shut down the robot and haul it inside the Cetacean, but when it reactivates, it will destroy everything in its path in an effort to escape.

The world is endangered when a vortex opens in the ocean, and Mark must find where its going and some how stop it before all the water is drained away.