Highlander: The Series (1992)

S01E03 Road Not Taken

Richie's friend falls victim to a lethal herbal drug, prompting MacLeod to investigate its origins. He suspects that another Immortal, Kiem Sun, is behind it, as Kiem has been attempting to refine the drug to eliminate its dangerous side effects for humanity's sake. However, one of Kiem Sun's followers is exploiting the drug for personal profit, leading MacLeod and his old ally on a chase to find him. Realizing that Kiem Sun intends to persist with his experiments, MacLeod decides to destroy the last remaining batch of the drug.