On their mission to explore the Mariana Trench at the very bottom of the ocean, the deepest and most inhospitable place on Earth, the crew of the deep sea vehicle Erebus make an unusual and startling discovery. A battered blue police box. As the Doctor, Romana and K9 join them on their journey, the submariners soon discover that the TARDIS is not the only unusual find lurking on the sea floor. Super-intelligent squid, long-lost submarines and their miraculous occupants are only the start of their troubles. The Goblins are coming. And they won't let anyone out alive.