Happy Valley's James Norton and Sally Wainwright tease explosive series ending

Happy Valley S3: Catherine Cawood (SARAH LANCASHIRE). (BBC/Lookout Point/AMC/Matt Squire)
Happy Valley season three is definitely the final instalment. (BBC)

Happy Valley is set to return for its final season after six years off air, and its creator and stars have teased a big finale.

The BBC drama, written by Sally Wainwright and starring Sarah Lancashire and James Norton, is set to come back for series three on New Year's Day, when it will end the harrowing story about police officer Catherine Cawood and criminal Tommy Lee Royce, who has changed the course of her family's life.

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Norton, who plays the criminal who raped Catherine's daughter leading to her taking her own life, said that just like the show's fans he had spent years trying to predict how the story ends.

Happy Valley S3: Tommy Lee Royce (JAMES NORTON) (BBC/Lookout Point/AMC/Alex Telfer)
James Norton plays Tommy Lee Royce. (BBC)

He said: "I think the reason this series is particularly exciting is because everyone knows it’s the last and so everyone is going to be waiting for something to happen, and everyone is sort of predicting and guessing how Sally wants to end it.

"I have been predicting for the last seven years how she is going to end it so it was really wonderful to read the script and hear her ideas - and they don’t disappoint."

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Wainwright explained that a large part of the reason for a long gap between series was to move on the plot for Ryan, Catherine's grandson born from the rape of her daughter.

She said: "I waited six years because I wanted to get to a point where Ryan would be old enough to start making choices about whether he wanted to have a relationship with his dad or not. And could he have a relationship with his dad, and how would Catherine feel about that? I really wanted to be able to explore that.

Happy Valley S3: Ryan Cawood (RHYS CONNAH). (BBC/Lookout Point/AMC/Matt Squire)
Rhys Connah is back as the now-teenage Ryan. (BBC)

"It’s been great that we got Rhys (Connah) back to play Ryan which has been fantastic, and he has done a really lovely job in that.

"That was always the intention, to have a gap and it has worked out just about right. Just the right period of time because he is now 16, so he can travel places by himself, he can make choices. He can do things behind Catherine’s back."

Wainwright added that there would not be any more Happy Valley: "The intention developed through conversations I had with Sarah (Lancashire) to make it a three-parter, to make a trilogy. We always said this would be the final season and it is very definitely is the final season."

Happy Valley returns to BBC One at 9pm on New Year's Day.