Happy Valley star James Norton says he hasn't even told his own dad how the drama ends - despite the fact his father also makes an appearance in the hit BBC show.
The ending is a secret the actor has kept close to his chest since filming the final series last year.
"My dad is the worst holder of any secret, so I fiercely kept it away from him, even though they were on set as extras," Norton told the BBC.
"They come on as extras occasionally and they came to Hebden Bridge with their dog, which was fun. But no, I've kept it pretty close to my chest.
"Also, I think no-one really wants to know. I've rarely found someone who knows and loves the show, who is keen to hear what happens."
While viewers will have to wait for the last ever episode to air on 5 February, Norton did tell BBC entertainment correspondent Colin Paterson he thinks it is incredibly written.
Both Norton and lead actress Sarah Lancashire were given copies of the script first, which they remained tight-lipped about until the rest of the cast and crew started to be given access.
"Everyone I've spoken to who has read it is like 'phew'," he said. "It's just great. It's Sally [Wainwright, writer] letting rip in her wonderful way. Going against expectation, but also giving you what you want. I loved reading it.
"There's also a little bit of melancholy tinge there because it's the end of a very significant chapter for everyone."
The show, which began in 2014, has won two Baftas for best drama. It is a series Norton knew was "special" from the start.
Filmed in the Calder Valley, West Yorkshire, the police drama delves into the life of sergeant Catherine Cawood (Lancashire), who is haunted by Norton's violent criminal Tommy Lee Royce.
After filming the first two seasons, the cast had to wait for seven years so young actor Rhys Connah, who plays Tommy's son Ryan, could grow older, allowing the story to develop.
"I think people change in seven years," Norton said. "Tommy obviously has a lot of trauma.
"We've looked a lot into his mental health and what it is that drives him forward. Tommy has moved on and grown and matured, and in some ways has calmed. He has this Jesus complex. He's a new man."
In many ways he is. His relationship with Ryan develops, letting the audience see his softer side. His motivations are no longer revenge and hatred, but something Norton characterises as a "narcissistic" version of love.
The actor described the scenes filmed with Connah as "incredibly charged" because of his character's growth.
But the darkness is still there, which Norton said makes the role so much fun.
"I've loved every twist and turn of this character," he continued. "There's something troubling about being in a very disturbed mind like Tommy. You have to delve a lot into psychopathy and spend time in the headspace of a man who's very mistrustful and quite a sad, angry person.
"But there's this other side of it where you feel kind of invincible because he doesn't care what people think about him.
"There's something incredibly empowering about that playing the bad man - intriguing, always playing the kind of subversive dark criminal mind."
The series airs on Sundays at 21:00 GMT on BBC One and BBC iPlayer.