The third instalment in the drama about the disappearance of a teenage girl, which starred Juliet Stevenson, captured a 19.3% share of the audience between 9pm-10pm.
The opening episode pulled in 6m viewers, while the second instalment was watched by 4.5m viewers.
However the primetime slot was won by BBC One's fly-on-the-wall documentary 'Motorway Cops - A Traffic Cops Special', with 5.1m viewers and a 22.4% share of the audience between 9pm-10pm.
The fourth episode of 'Amazon with Bruce Parry' brought BBC Two 2.5m viewers and an 11% share in the same slot.
Channel 4 documentary strand 'Dispatches' took an in-depth look at the erotic entertainment business and its growing links with prostitution in 'The Hidden World of Lap Dancing'.
The programme had 1.2m viewers and a 5.3% share of the audience in the 9pm-10pm slot.
Five's 'Unbreakable', a jungle warfare-style reality series, launched with 898,000 viewers and a 3.8% share between 9pm-10pm.