Among the greatest dramatic triumphs of Slow Horses Season 4 is its unflinching depiction of David Cartwright's cognitive decline, brought to life through Jonathan Pryce's extraordinary, poignant performance.
Known throughout British intelligence as the legendary 'OB' (Old Bastard), David Cartwright shaped decades of MI5 operations during the Cold War. Yet in Season 4's opening episode, his paranoia blurs with dementia. Convinced that assassins have entered his country home, Cartwright fires a shotgun through his bathroom door, inadvertently setting off a nationwide manhunt and dragging River into a lethal conspiracy.
Pryce captures the tragic vulnerability of an elite mind deteriorating: moments of sharp, imperious authority instantly collapsing into childlike confusion. The dynamic between Pryce and Jack Lowden provides the series with its deepest emotional resonance. River’s desperate drive to protect his grandfather—even as Cartwright's buried operational missteps put everyone in danger—proves that loyalty among spies is often the heaviest burden of all.