STRIKE: Troubled Blood viewers were tonight left with their mouths open after the killer was finally revealed.
This evening fans tuned into the gripping whodunnit BBC One drama, to watch the season 5 finale.
The new four-part series kicked off last week, and saw the return of Cormoran Strike and his detective partner Robin Ellacot.
They were called in to solve their first-ever cold case involving a young doctor and mother named Margot who disappeared in the 70s.
In Troubled Blood, Cormoran and Robin grapple with several suspects who could have been involved in Margot’s disappearance.
This included a psychopathic serial killer who had murdered several women around the same time, a former patient of Margot’s whose future girlfriend was also found dead, and a friend’s abusive husband who Margot protected her from.
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But in a shock twist, that no one saw coming, the REAL murderer was revealed - who fans didn't suspect.
It turns out that the culprit all of those years ago was Janice Beattie, a nurse at Margot’s practice.
She would regularly poison her friends and family if they bothered her over the years - and subsequently murdered several others for the fun of it - including her young son.
After realising that Margot knew what she was up to, Janice drugged and killed her.
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She hid her in a cement concrete container hidden in a house, which was subsequently found by Robin.
Fans loved this HUGE twist, and all said they didn't realise Janice was the murderer, with this one tweeting: "Blimey, who'd have thought it #strike."
While another said: "I am in complete shock! One of the best twists , I have ever seen! The little old lady! #strike."
This viewer tweeted: "That is a great twist! I couldn't believe how much that little old lady loved killing! #strike."
This one commented: "Bloody hell, she’s a nasty piece of work! #Strike."
Meanwhile, on a brighter note, the season concluded with Strike and Robin sharing a sweet moment at the end, when he forgot her birthday and bought her a balloon some perfume.
Fans are now hoping the pair will get together in the next series.
Strike is based on the book series Cormoran Strike by J. K. Rowling under the pseudonym Robert Galbraith.
You can watch all of season 5 of Strike on the BBC iPlayer right