The Private Army of Colonel Stone - S1.E5

The episode opens in Africa where three men who appear to be close to death are rescued. Then Rupert's elderly 'honorary aunt' Rachel writes to tell him that her son Jamie has died out in Africa and later rings to say she's been told that there's a man living at her son's cottage. The man is Colonel Stone who ran the expedition that Jamie was on when he died. He describes himself as a friend of Jamie's but Aunt Rachel says that Jamie told her that he didn't like the Colonel. Rupert and Amy decide to investigate.