While Maya Cobbina (Sophie Okonedo) prepares to accept a top job as the first black Director of Public Prosecutions, it gives her the chance to investigate an old case of a death in police custody.
However, there are powerful people who don't want her to learn the truth.
Meanwhile, her husband Nick (Adrian Lester) is sickened by his act of betrayal, which leads him to reflect on the actions that led him to this point.
The episode goes back in time as we join Nick in 1996 as a fearless undercover officer who is left furious when his work is undermined by a radical young lawyer.
Soon enough, on his next assignment he finds himself face-to-face with the lawyer, Maya, where soon enough he struggles to keep the professional and the personal separate when his feelings for her develop and he questions his own beliefs within the police force.
Undercover, 9pm, BBC One
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