Claude Whelan is back. And for anyone who watched Diana Taverner manage her former boss’s limitations in Slow Horses, the news has a particular appeal: the man who seemed to need instructions to survive his own job could become a problem for her. James Callis is confirmed for the seventh season, which adapts Bad Actors, the eighth novel in Mick Herron’s series.

The announcement also adds five names to the cast: Varada Sethu, from Andor and The Other Bennet Sister; Ethan Moorhouse, from Ludwig and The Moment; Tuppence Middleton, from Mank and Downton Abbey: A New Era; Merab Ninidze, from Homeland and The Courier; and Joshua McGuire, from The Gold and Anatomy of a Scandal. Their characters have yet to be revealed. Gary Oldman continues as Jackson Lamb, now facing a mission to find a mole at the heart of the British government before they can bring down the state.

It is a premise that promises to explore one of Slow Horses’ greatest strengths: the difficulty of distinguishing a threat to national security from an operation designed to protect someone’s career. And the book offers particularly interesting material for Kristin Scott Thomas.

Spoilers for the plot of Bad Actors follow.

In the novel, the disappearance of Sophie de Greer, a forecasting specialist working for a Downing Street think tank, triggers the crisis. She was recruited by Anthony Sparrow, an adviser to the prime minister who wields considerable power behind the scenes and intends to extend his control over the intelligence service. The possibility that he has placed a Russian mole so close to the government is a disaster he needs to manage — preferably by shifting the blame.

This is where Whelan receives his assignment to investigate. The trail leads to MI5 and places Diana under suspicion. His return therefore serves a much more interesting purpose than simply bringing back the boss she could manoeuvre: he is now part of an investigation capable of compromising her position.

The dispute escalates to the point of threatening the service’s independence. Diana needs to protect herself, and Lamb becomes an ally as inconvenient as he is necessary. After so many barbed exchanges, there is pleasure in imagining the two forced to acknowledge something that has always been there: however incompatible their styles may be, they know the job and recognise each other’s competence. If the adaptation preserves this thread, Scott Thomas will have plenty of room to explore Diana outside the safety of her office.

As for the newcomers, we can already start piecing the puzzle together, with the caveat that none of these character assignments have been confirmed. Tuppence Middleton would be a plausible choice for Sophie de Greer, the woman whose disappearance drives the story. Joshua McGuire, meanwhile, could take on Anthony Sparrow, a character requiring a mixture of authority, opportunism and an ability to make any room unpleasant.

Varada Sethu would be a possibility for Ashley Khan, an agent who arrives at Slough House after a failed assignment involving Lamb. In the book, he realises she is following him and breaks her arm. She is subsequently demoted and blames him for destroying her career prospects. In other words, another person entering that building with reasons to hate the boss.

Ethan Moorhouse could play Lech Wicinski or take on a role reworked by the screenplay, but that is a less certain fit. The strongest theory concerns Merab Ninidze: he has already voiced Vassily Rasnokov in season six, and the Russian character appears in Bad Actors. His casting suggests the possibility of an expanded presence.

When it comes to the returning cast, some caution is needed. The general list of the show’s actors reproduced in news reports does not confirm all of them for season seven. In the book, Diana, Catherine, Louisa, Roddy and Shirley remain part of the story, but that does not replace an announcement about their performers.

River deserves particular attention: he remains outside the main action of Bad Actors as a consequence of the attack he suffered in the previous novel. This does not confirm Jack Lowden’s departure, much less determine how much television might show of the character. Olivia Cooke’s return as Sid has not been confirmed in this announcement either, but the character appears in the book as River’s girlfriend, offering a way for both to remain part of the adaptation.

For now, the most delicious certainty is Whelan. Diana may have got rid of him at the top, but Herron knows that a former boss can keep causing trouble long after losing his chair. Especially when someone discovers how to exploit his desire to matter again.