December 1984. SIS field asset Heidi Sperling [codename Thistle] exfiltrates from East Berlin with the sole copy of a critical intelligence leak: a naval log containing a solitary message received from a previously unidentified Soviet 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 whole years. And now this latest message reads: ACTIVE.

Picked up by the Royal Navy’s submarine HMS Viking, Heidi is thrust into a 'black ops' mission: find TK15 and neutralise it at any cost. But as her only ally on board the Viking falls sick, she realises this modified vessel is far more than a Soviet experiment to gain an upper hand in the nuclear arms race. Here, in the crushing depths of the North Atlantic, it seems something darker has awoken - something that cannot be contained by any superpower.

As Heidi’s own reality twists around her, an unknowable force cripples the Viking's defences and drives its crew to madness. Trapped in the deep, Heidi has no choice but to find a way to save the remaining crew and stop TK-15 for good, before it steals what’s left of her mind . . .