Captain Mike Yates is investigating the disappearance of artefacts from an archaeological site deep below Dunstanton Lake. It’s hardly a job for UNIT. But when the team discover a mysterious ancient structure buried deep underwater, all that changes.

When chief archaeologist Freda Mattingly ventures inside, she soon realises that her skills do not begin to equip her to deal with what she finds. As an ancient menace begins to stir the Doctor, Jo Grant and Mike Yates must dive down to the lake bed and discover the secrets hidden there. Secrets that could mean the end of all life on Earth…