Episode 9

Eight customs and excise officers are murdered, and MI5 is suddenly waist-deep in the Colombian drug trade. Tom had been undercover with Customs, and when he steps away to phone in, the rest of his crew are murdered. The trail leads to the Chala Cartel, the country's biggest trafficking outfit, and from there to somewhere worse: surveillance links the cartel to one of Britain's leading oil companies. The corruption runs higher than anyone in the building would like to admit, and the bodies are the proof of it.

Zoe and Tom are sent undercover to turn the girlfriend of the man who runs the cartel. It is delicate work at the best of times, and these are not the best of times. Tessa Phillips is back, and she wants revenge on Harry and on MI5. Her grudge is personal, and personal grudges burn through operations faster than any enemy can. The hour keeps asking whether Tessa will get her payback before the mole inside the service is found, and whether Tom and Zoe will end up as casualties of a quarrel that isn't theirs.

Jenny Agutter plays Tessa with the steel the role always needed. Santiago Cabrera's Camilo Henriquez and Daniel Cerqueira's Rafael Morientes give the cartel a real presence, while Megan Dodds's Christine Dale, Michael Cochrane's Ross Vaughan, Abigail Hercules, Sophie Lombard and Shauna Macdonald fill out the ensemble. Peter Firth is Harry, Keeley Hawes is Zoe. Ben Richards and David Wolstencroft wrote it; Sam Miller directed.

The drug war material is competent, but the returning grudge gives the episode its bite. Watching Tessa work her way back into a world that threw her out is more absorbing than any amount of cartel intrigue. And the final act keeps the mole, the cartel and one very determined woman in the air until the last scene, which is a neat trick for an hour that could have settled for a straightforward bust.