Tense submarine-set drama Vigil draws to a close this weekend, with Suranne Jones' DCI Amy Silva last seen facing being torpedoed into the deep sea.
Whether or not she survives her watery nightmare is just one of a whole host of questions we need to have answered by the time the final credits roll.
International espionage, nerve agent warfare, nuclear weapons and a mysterious love story turned sour all have loose ends waiting to be tied up come Sunday evening.
We round up our burning questions ahead of the sixth and final episode of Vigil.
BEWARE: Contains spoilers for Vigil episodes 1-5
Will DCI Silva get out of the torpedo tube?
Straight in with the biggest question facing us in the finale - will DCI Amy Silva (Suranne Jones) manage to get out of the torpedo tube, or is she facing down the watery, confined death of her nightmares?
Episode five ended on the biggest cliffhanger so far as CPO Matthew Doward was pinned as the probable crew member spying for the Russians shortly before knocking out Amy and packing her into the tube ready to be torpedoed into the sea along with some incriminating evidence.
Read more: The best TV to watch this autumn
In terrifying scenes that have been haunting viewers, Amy came round just in time to realise what had happened and the episode cut to a close.
However, given that the drama killed off Martin Compston's character Craig Burke in the first episode, it's possible that Amy could be ruthlessly removed, too.
What went wrong between Kirsten and Amy - and will they get back together?
The series so far has been dotted with flashbacks to Amy's life back on land and past relationships - with her partner who died in a car crash into a lake, and later with DS Kirsten Longacre (Rose Leslie) after they began working together.
Although Kirsten's tireless efforts to help Amy's submarine mission back on land and visit to her daughter with a birthday present show she still cares for her ex, there have been moments of definite tension between them.
DSU Colin Robertson, Amy's boss, seemed shocked that she chose Kirsten to work with on the case, while Amy's late partner's parents had thought they were still together, so there must be plenty of backstory involved.
Will the threat to Amy's life on the case bring them back together, or is there too much water under the bridge that we don't yet know about?
Vigil's final episode airs on BBC One at 9pm on Sunday, 26 September
How much of a threat was Burke to the Russians?
When Burke was killed off in episode one, Amy's immediate assumption was that he must have held some potentially explosive information about the Russians for them to have had him assassinated with a nerve agent.
However, when it became clear last week that cook Jackie had killed Burke so that the Russians could get their own replacement man, Doward, on board in exchange for freeing her son from jail, the emphasis shifted.
Could Burke just have been unfortunate collateral so that Doward could gain access to HMS Vigil?
Or is whatever information he encrypted in hidden files for murdered girlfriend Jade threatening enough that he was a serious target for the Russians - and what was it?
Why is Ben Oakley at the Chinese embassy and what did Patrick Cruden know about Doward?
Rumbled Ben Oakley went on the run last episode after police discovered he was spying for the Russians in the anti-nuclear war camp.
He managed to pull the wool over the eyes of MSP Patrick Cruden (also secretly Jade's dad) long enough to get some help in hiding out, but why did he make the surprising choice to head for the Chinese embassy rather than the Russian one? It poses questions about China's involvement in the intrigue.
Read more: Endeavour series 8 - all you need to know
Part of Ben's bargaining to get Patrick's help hinged on a photo he had of Russian ambassador Peter Ingles with the asset on the submarine, who now appears to be Doward.
The photo could have spelled the end of nuclear weapons in Scotland if he had released it, so has Patrick had a bigger part in, and more knowledge of, the murky dealings all along?
Was Doward the only saboteur?
Until Amy and now-doomed coxswain Elliot Glover (Shaun Evans), who turned out to be a red herring bad guy, worked out Doward's involvement, he had been a relatively minor character.
His most notable scene until then had been flying in to the Vigil drop off point with Amy when she first boarded the boat, telling her he shouldn't be speaking to her.
Could Doward really be the mastermind behind the entire plot? It seems more likely that there are further levels above him of Navy officers spying for the Russians and that he is every bit as disposable as Burke and Jackie.
What is the 'bold' final twist?
Show star Suranne Jones has said: “It’s a bold show, with two female leads. It has a romantic story but the thrills that come to the last episode were amazing and quite shocking.”
Obviously, that hints at a further big twist to the potential torpedoing of her character, but whether it's a shock death or an espionage plot reveal remains to be seen.
All we know is, we'll be tuning in at 9pm on Sunday to find out.
Watch: Kit Harington offers rare glimpse into life as a new father