Slow Horses season 4, episode 3, “Penny for Your Thoughts,” finally reveals a bit more of the mystery plaguing River Cartwright (Jack Lowden), one of Slough House’s MI5 agents, and his grandfather, retired MI5 agent David (Jonathan Pryce). Previously, Slow Horses confirmed what happened to River. After David killed his grandson’s look-alike and his would-be assassin, River used the dead body to his advantage. Planting his ID at the scene and disfiguring the intruder’s face, River chased his only lead to Lavande, France. Thankfully, Jackson Lamb (Gary Oldman) doubled down on River’s ruse, allowing him more time.
https://screenrant.com/slow-horses-season-4-episode-3-recap-story-reveals/