Episode five, titled 'Gimme Some Truth', pulled in 2.66m (11.1%) for Five, compared to Murderland's 5.49m (22.8%).
The 9pm show was down slightly from last week's instalment, which also lost viewers to the Robbie Coltrane-fronted drama.
Also at 9pm, BBC One's Life had 5.27m (21.9%), 1.26m (5.2%) watched The World's Greatest Money Maker: Evan Davis Meets Warren Buffett on BBC Two, and Channel 4's Race And Intelligence: Science's Last Taboo managed 1.02m (4.5%).
University Challenge and Restaurant In Your Home logged 3.36m (13.8%) and 1.61m (6.7%) for BBC Two in the 8pm hour, while The Gadget Show took 1.35m (5.6%) for Five, and 1.55m (6.4%) watched Dispatches: Do You Know What's In Your Breakfast? on Channel 4.
BBC One's Panorama interested 2.55m (10.6%) at 8.30pm, and ITV1's half-hour Make Me An Apprentice: Tonight had 2.75m (11.3%) at 8pm.
Overall, ITV1 came top during primetime with an average share of 25.5% to BBC One's 21.4%. BBC Two was third with 7.3%, followed by Five's 6.1% and Channel 4 with 4.6% (+1: 0.7%).
Over on the multichannel stations, 278k (1.2%) tuned in for One Tree Hill on E4.
BARB ratings data supplied by Attentional