Somewhere in northern England, two men out fishing in a boat are pulled underwater by a hand capsizing their boat. Paul Foster is briefed to investigate other disappearances in the area, where four UFOs have landed but no Aliens have been found. Taking a Mobile, he and operative Maine are in the area when another UFO arrives. A snow-covered moorland ditch tips their Mobile and cripples a track, and by the time Foster and Maine find the UFO it has burned out. But the snow shows tracks by Aliens, leading to a river. Splitting up to search, Maine is pulled underwater by Aliens, and taken to an underwater base in a cave. Recognising Maine is from SHADO, the Aliens take their captives in air cocoons down river under the surface. By now a second Mobile has found Foster, and he dives into the river to investigate. With the Aliens gone, Foster guesses they are making for the estuary, and warns Skydiver to head them off. But on the river bed, the Aliens open fire as the Mobile approaches, and only Foster gets out alive. Using a reed to breath, Foster plays dead in the water until the Aliens move on, and warns Skydiver. At the estuary, Carlin detects a craft approaching, and ambushes it as it tries to pick up the Aliens. The human captives, protected in their cocoons, are fished out of the water, to hopefully be revived by SHADO scientists.