VIGIL viewers described feeling 'tense' and 'claustrophobic' during the series finale.
The BBC drama follows mysterious disappearance of a Scottish fishing trawler and a death on-board a Trident nuclear submarine brought the police into conflict with the Navy and British security services.
DCI Amy Silva lead an investigation on land and at sea, with the action cutting between the investigation in Glasgow, Scotland and the unfolding drama at sea.
Surrane Jones played DCI Silva on the BBC drama set on a submarine and started the episode still stuck in the torpedo tube as it filled with water.
DCI Silva struggled to breathe as the water filled around her, when some submariners luckily drained the tube of the water via a computer screen, it still didn't mean rescue for her.
She kicked the door and screamed for help as the detective struggled to get oxygen in.
Silva dipped in and out of consciousnesses as she remembered a dramatic fight between her and partner DS Kirsten Longacre (Rose Leslie) who was also investigating the murder from the ground.
She whispered, "come back" as she saw in the flashback, Longacre storming out of her home.
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Silva eventually blacked out with viewers feeling very worried for her safety.
"This episode is making me hold my breath & it’s stressing me out! #vigil," wrote one fan on Twitter.
Another added: "Never felt as claustrophobic as I do watching #Vigil."
And a third wrote: "I don’t know why I do this to myself and watch #Vigil it gives me the worst anxiety."
Viewers were shocked again just moments later when Lieutenant Commander Mark Prentice (Adam James) rescued Silva and dragged her out of the tube.
But as he tried to get the very poorly Silva on her feet to raise the alarm about the killer, he turned up to silence them.
Matthew Doward (Lorne MacFadyen) confronted the pair, as Prentice ordered Silva to run to safety.
The men started fighting, with Doward getting the better of his superior and stabbing him multiple times as a helpless Silva looked on.
Fans watching at home could not believe they were witnessing such a graphic murder.
"I don’t think I’ve seen a murder so graphic before #Vigil . I don’t want to see another," wrote one person.
Jones was joined by Line of Duty star Martin Compston, who played an underwater expert.
Vigil was written and created by Bafta-nominated writer Tom Edge who is known for The Crown and comes from the same producers as Line of Duty.
As much as the gripping story will feel like it could be true, it is not based on real-life events.
Speaking at a press event for the show, Edge said: “I think over the next 10 years submarine warcraft is going to dominate in a way they haven’t been thought about for a while.”
The show will explore the ethical questions raised by the existence of a nuclear deterrent submarine programme, and the spiralling costs of submarines.
However, the peace camp in Vigil is inspired by the real-life Faslane peace camp.
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The camp is located next to Faslane Naval base in Scotland - the home of the Trident nuclear programme.
Vigil is available to watch on BBC iPlayer.
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