Episode 2

A dismembered body is discovered, and DCI Roz Huntley fails to report for duty. AC-12 cranks up its investigation into Operation Trapdoor, worried that Michael Farmer is going to jail for crimes he did not commit. The suspicion grows that Roz has held back key evidence, and a new DCI is drafted in to take over the operation. The wheels are starting to come off, and everyone can hear it. The missing informant and the missing officer are two holes in the same net. It went out in April 2017, early in the fourth series, and it keeps the screws on from the first scene, with Ifield's absence felt in every room AC-12 enters.

The episode is a study in how an investigation frays. Kate makes progress in her undercover role, but the unit's main informant, Tim Ifield, has gone missing, and without him the case loses its anchor. Further questioning of Farmer turns up new information that challenges the whole case against him. Every thread pulls at another, and the pressure cooker atmosphere does not let up. The show has always been good at this: the slow unravelling of a case that looked solid.

Jed Mercurio wrote it and directed it. Vicky McClure's Fleming does the delicate work undercover, while Martin Compston's Arnott and Adrian Dunbar's Hastings run the visible side. Thandiwe Newton keeps Roz coiled and watchful, with Mark Stobbart's Twyler, Henry Miller's Morganstaff, Claudia Jessie's Taylor, Maya Sondhi's Bindra, Vineeta Rishi's Pandit, Scott Reid's Farmer and Andy Kellegher's prison officer in support. It is a dependable ensemble, and the episode leans on it without strain.

The episode runs on the gap between what the case looks like and what the evidence says. Farmer's fate is the engine, and the missing Ifield is the worry underneath it. The show understands that an investigation is a fragile machine, and this one is coming apart in front of everyone. The new DCI running the operation sees the case with fresh eyes, which is exactly what nobody wanted. But nobody is quite sure whose fault it is, which is the point.