Hidden Truth

A London drug lord, Jack Flynn, has one of his own dealers murdered in Denton — a loose end to be snipped before trial — and the only witness is Cathy Thompson, a single mother placed in protective custody with her young son Robbie. Flynn's professional killer, George Starkey, attacks Cathy's home and critically wounds the WPC guarding her before fleeing. Frost, charged with keeping Cathy alive until she can testify, calls in DS Reid for his Metropolitan Police connections, and the episode becomes a siege in slow motion.

Flynn, always a step ahead, hires a second, younger hitman as a decoy to pull the police off Starkey. The distraction works: armed response officers spot the newcomer, and one of them, Russell, is killed pursuing him. Frost, examining the shooting, reaches a terrible conclusion — the precision of the gunfire means Russell was killed by his own partner, Hadley. The motive is a cliché with a corpse in it: Hadley's wife was having an affair with Russell. Hadley hangs himself in his cell of guilt.

Starkey attacks the new safehouse — a location only a handful of people know — and in the chaos, Cathy makes her choice: she runs, taking Robbie with her, unwilling to keep risking their lives to put Flynn away. Frost, with Reid's help, finds the leak: Eileen Cleary, who sat on the prosecution team — and had borne Flynn a son six years earlier. And in reviewing Cathy's statement, Frost discovers the hidden truth at the episode's centre: it was Robbie who witnessed the murder. Cathy took the burden herself to protect her son.

There is no time to be angry. Cathy has agreed to meet Cleary at an air museum — a trap — and Frost and Reid race there as Starkey closes in. They capture him; Robbie testifies; Flynn falls. In the episode's final, quietly devastating beat, the Denton officers find the lottery ticket Frost lost — worth £5 million, his numbers right — but it is out of date. Hidden Truth, written by David Gilman and directed by Paul Harrison, is the series at its most relentless: a chase thriller about witness protection that keeps finding new ways to remind its hero what protecting people actually costs.