Ṣọpẹ Dìrísù joined the Slow Horses cast in season 3, which followed a subplot of his character's relationship with Alicia Dunn, who met a tragic fate due to some organizational disputes.
The Gang of London star became part of this show, which centers around a group of failed or disgraced MI5 agents who are sent to Slough House to work on the assigned tasks. British comedian and screenwriter Will Smith has been a showrunner on this spy thriller, which is based on Mick Herron's eponymous novel series.
Gary Oldman headlined the show as Jackson Lamb, with Jack Lowden as River Cartwright, Saskia Reeves as Catherine Standish, and Rosalind Eleazar as Louisa Guy, who were part of his Slough House. Kristin Scott Thomas's Diana Taverner has also been central to the story, and the show has followed her strained professional relationship with Lamb.
Slow Horses featured some guest performers, and Sope Dirisu was one of them. However, his character, Sean Donovan, met a tragic fate by the third season's finale. So, he won't likely return to play the part in any of its future seasons. Yet, before diving into the show's new season, if you want to know Donovan's subplot, here's all you need to know.
In Slow Horses, Sope Dirisu played the former head of security at the British embassy in Istanbul
Sope Dirisu was introduced in the show's third season in Istanbul as Sean Donovan, the said executive, who was in a relationship with MI5 agent Alicia Dunn (Katherine Waterston). However, Donovan was told to secretly investigate Dunn for her involvement as a potential whistleblower. She was accused of stealing a classified Footprint file, which could have exposed one of the agency's dark secrets.
While in Dunn's apartment, Donovan started looking for the same file, but got caught and confronted by her. He came clean about his motives, but she didn't. Later, she left the house to hand over the same file, and he followed her. However, it didn't work out well for her, as he found her dead. It left him in a dilemma where he was unsure whether to prioritize his personal regrets or his professional duties.
Eventually, Donovan tried to get in touch with the Slough House administrator, Catherine Standish (Saskia Reeves), by meeting her at an AA meeting. Later, with the help of Alicia's siblings, Ben and Sarah, he kidnapped her and took her to a remote location to meet Chieftain Security operative Sturges (Nick Blood).

Soon, after Sturges ran away from the Dunn family home, Donovan went to the MI5 storage facility with Ben for the Grey Books. That's where they met River Cartwright (Jack Lowden) and Louisa Guy (Rosalind Eleazar), and unlike what they claimed, they started looking for the file that got Alicia killed. Donovan read the file and was shocked by its contents.
By the end of Slow Horses season 3, Donovan realized that Alicia had been killed for the same reason he suspected. However, within moments, the Chieftain operatives bombed the location and killed Dunn. After his death, Donovan tried to escape with Guy's help, but was left wounded and potentially dead by a gunshot. That's why he won't likely return in Slow Horses season 4.
Also read: What happened to Nick Duffy in Slow Horses? Character arc explored