When the TARDIS lands in the sea off the eastern coast of England, the Doctor, Jamie, and Victoria investigate a nearby beach, which seems to have an improbably large amount of sea foam as well as a major gas pipe marked “Euro Sea Gas”.
When the Doctor examines the pipe using a sonic screwdriver, he thinks he hears a heartbeat from within. The trio are captured and put in a cell by Robson, a ruthless gas refiner who heads a pumping operation with a network of rigs spanning the North Sea. His second-in-command is Harris, a scientist.
Robson is unnerved by the loss of contact with gas drilling Rig D at sea, plus an unexplained drop in the feed line from the rigs. The Doctor suggests that the supposed heartbeat could be a creature inside the pipe and suggests that the gas flow be suspended while he investigates, but Robson refuses to do so, and has Harris lock up the travellers.