Tonton Streaming Highlander: The Series (1992) S1E3 - Road Not Taken

A deadly herbal drug takes the life of Richie's friend, and MacLeod suspects the source of the drug is another Immortal, Kiem Sun, who has long sought to perfect such a drug and remove its lethal side effects for the betterment of mankind. But a disciple of Kiem Sun's is using the drug for his own gain, and MacLeod and his old friend must track him down. When it becomes clear that Kiem Sun plans to continue his experiments, MacLeod destroys the remaining sample of the drug.