The Swedish crime drama adaptation, starring Sir Kenneth Branagh, fell to 4.44m (20.4%) in the 9pm hour, down 800k week-on-week and a sizeable 2m on its premiere a fortnight ago.
Elsewhere on the channel, Casualty's weekend double bill concluded with 4.38m (20%) at 8.10pm, prior to which Countryfile nabbed 5.03m (25.5%).
Meanwhile, Engineering Giants (1.44m/6.5%) and World's Most Dangerous Roads (1.67m/7.5%) delivered decent ratings to BBC Two.
The ever-dependable Come Dine with Me served up 1.22m (5.6%) for Channel 4 at 8pm (+1: 159k/0.7%), then Thelma's Gypsy Girls logged 1.64m (7.3%) from 9pm (+1: 384k/2.1%).
Once Upon a Time dipped to 869k (4%) for an episode titled 'Hat Trick' on Channel 5 (+1: 184k/0.8%), and Big Brother grabbed 1.24m (5.7%) as well as 155k (0.8%) on timeshift.
Overall, BBC One bagged a convincing primetime win with 22% against ITV1 which averaged 14.3% (+1: 1.1%). BBC Two and Channel 4 tied in third place with 5.8% (C4+1: 1.6%), ahead of Channel 5's 4% (+1: 0.6%).
Family Guy topped the multichannel ratings with 954k (4.7%) and 965k (5.4%) for its late-night double bill. The Only Way Is Essex returned with 935k (4.9%) for ITV2 at 10pm (+1: 192k/1.8%).
BARB ratings data supplied by Attentional