December 1984. SIS field asset Heidi Sperling [codename Thistle] exfiltrates from East Berlin with the only copy of a critical intelligence coup - a naval log containing a solitary message received from a previously unidentified Soviet Typhoon ballistic missile nuclear submarine.

Incredulously - impossibly - it seems the vessel, known only as TK-15, has been sitting motionless and undetected in the waters between Scotland and Iceland for three years. Now NATO needs to be on high alert because that one-word message reads: ACTIVE . . .

Picked up from East Germany's Baltic Coast by the Royal Navy's hunter-killer submarine HMS Viking, Heidi is thrown into a mission to find and investigate the TK-15, and must confront her own paralytic fear of the ocean's crushing black depths and HMS Viking's seemingly hostile crew. When her only apparent ally, Executive Officer Daniel Vickers, disappears as they investigate the TK-15, she realises this modified submarine is far more than a Soviet experiment to gain a strategic upper hand in the nuclear arms race. Here, at the bottom of the ocean, the Soviets have - for good or ill - delved too long and too deep into what lies beneath and woken something far, far worse.
As Heidi's own reality twists around her, as an unknowable force drives the crew to madness and cripples the British submarine's defences. Can Heidi control her own escalating fears and help bring the Soviet craft to the surface? Or will she make the ultimate sacrifice in order to stop the mysterious TK-15 from completing its dark and terrible mission?