The exploits a team of people whose job is to investigate the unusual, the strange and the extraterrestrial.

In the dark silence of the sea during World War II, the submarine USS Tiger Shark prowls on what should be a routine rescue mission. But for the shell-shocked crew, trapped together in the sub's narrow corridors and constricted spaces, this is about to become a journey into the sensory delusions, mental deceptions and runaway fears that lurk just below the surface of the ocean.

Set during the 1960s in the fictional North Yorkshire village of Aidensfield, this enduringly popular series interweaves crime and medical storylines.