Parts: 6
Writers: Victor Pemberton
Airdates: 16 March 1968 - 20 April 1968
Production Code: RR

Doctor: second
Companions: Jamie, Victoria

Creature Feature: Weed

Story: The TARDIS makes a splashdown in the North Sea just off the English coastline. After noticing the unusual amounts of foam along the seashore, the Doctor, Jamie and Victoria come across a large pipeline emerging from the sea. Curious, the Doctor opens it with his sonic screwdriver, and all three hear an odd throbbing sound like a heartbeat coming from within the pipe. As they turn to leave, shots are fired, and Victoria watches with horror as the Doctor and Jamie are both hit, and she is next...

Their examinations of the emergency release valve have aroused the suspicions of the crew of a natural gas control rig, and when they awaken from the tranquilizer dart - relieved to be alive - they learn that they are now aboard the rig. Despite the fury of the controller, Robson, the TARDIS crew manage to befriend his second-in-command, Harris, who tells them that the gas pressure has been dropping and that they have lost contact with one of the rigs. Robson is eager to find the cause, and would be happy to blame it on the Doctor.

But the Doctor is suspicious that the problems at the rig are being caused by something other than mechanical error or sabotage. And when Victoria is attacked by a horrific seaweed creature, his suspicions are confirmed. It seems that something is down in the darkness of the impeller shaft, waiting for the right moment to attack the control rig...and the Doctor has no idea how to stop it.