Peter Firth reveals the shocking secrets to come in the final series of BBC One’s Spooks

When we last saw spy boss Harry Pearce he was suspended after handing over state secrets to save Ruth’s life…
“Yes, and he’s been on gardening leave ever since. Thankfully, he survives his tribunal and he’s reinstated in the counter-terrorist Grid when a situation arises that needs Harry’s experience.”

So what’s happened?
“The UK and Russia are trying to form a new partnership, but an old spy colleague of Harry’s from his days in Berlin during the Cold War is murdered. This coincides with the arrival of his old adversary, Ilya Gavrik. He’s a Russian politician who is married to Elena, an old flame of Harry’s.”

Did Harry sleep with the enemy during the Cold War?
“No, Elena was actually a double agent – she was spying on the Russians for Harry. Her son, Sasha, discovers evidence that suggests she’s still spying for Harry, and it leads to a very dramatic revelation.”

Sounds complicated – what’s Harry and Ruth’s relationship status?
“The arrival of Elena and the revelation of a big secret from their past propels them into a new phase. Harry’s got a past life that comes back to haunt him, personally and professionally.”

This is the last-ever series of Spooks – are you sad to see it go?
“Each series has topped the previous one and this year is no exception. The guest stars we’ve had over the years have been incredible – I think I’ve worked with everybody in my profession! I’ll miss it, but not the getting up at 5am for filming.”

So does Harry go out with a bang?
“I can’t see Harry being killed off – but I don’t think he’ll be retiring to run a tea shop in Devon, either.”

Spooks returns on Sunday, September 18 at 9pm on BBC1