When the Kinloch Bravo's crew find themselves stranded on their production platform, first when their chopper is diverted and then by a mysterious fog that affects the entire oil field, rig boss Magnus' attempts to find a practical solution are thwarted by a horrifying accident and a growing sense of unease onboard.

The appearance of the rig's Stand By Vessel raises hopes of escape in the team – leaving geologist Rose until sunrise to try and work out the scientific nature of the mysterious ash onboard before the crew abandon ship and make a break for it. But with tensions running high, some are compelled to take the situation into their own hands, with serious consequences.

Baz and Garrow get closer to understanding their mission when they explore the pump control room, and realise they need Fulmer to help them – leading to painful consequences for Fulmer as he struggles to keep a grip on his sanity. When the crew's anxieties rise as a glimmer of potential hope turns sour, Magnus finds himself losing his grip as tensions amongst the group reach fever pitch.

With Baz missing and presumed dangerous, the crew split into groups to search the production module and find him, and Rose takes the painful decision to isolate Fulmer. But as Rose makes a perplexing breakthrough, the manhunt becomes more frightening and mysterious when the crew are hit by power failures and paranoia, and they discover things that push them far beyond their comfort zone.

The new arrivals on board from the Kinloch Charlie immediately make their presence felt, as Pictor head Coake enlists a group to undermine a weakened Magnus and force their way to the pump control. But when Coake's motives are investigated and the crew realise he isn't giving them the full truth, they find themselves locked in an urgent race for answers with lives at stake.

Held captive on a new high-tech rig, the Bravo crew are launched into a rescue mission deep beneath the sea. But Pictor have not revealed all they know about what lurks on the ocean floor...

Fulmer's rift with Rose leaves him open to manipulation from Coake, but his plans to exploit Fulmer are interrupted when Cameron takes a drastic step.

The Bravo team race against time to reach the Heart of the Ancestor before Pictor. But when they learn that Darian still has an ace to play, Rose and Fulmer have no choice but to give themselves over to a heroic act of self-sacrifice.

Laden with explosives, Magnus, Fulmer and Bremner head onto the Arctic glacier to bring aid to those trapped below.

After a shocking death on board, the team are tested to their limits by a deadly blizzard that brings them into close contact with the Ancestor – and another danger even closer to home.

Trapped in the Subsea Module, Rose and Cameron must stop Pictor getting the data drive. But an ambush on the Wet Dock leads to a gunshot that changes everything.

Scores are settled and mysteries revealed as Coake is pushed to explain Pictor's plan and the true reason for his mission onboard, Rose and Fulmer work out the horrifying extent of what the coded messages they have discovered really mean, and Magnus must put his personal demons to rest and lead the crew in one final bid for their survival.