S3·E20 Reasonable Doubt

When a valuable DaVinci sketch is stolen from a friend of MacLeod's in a robbery that killed two guards, MacLeod offers to act as go-between to ransom it back. He discovers the sketch was stolen by Kagan, an Immortal he faced once before, when Kagan was a bank robber in 1930 Paris and MacLeod killed his mentor. Meanwhile, Maurice asks MacLeod to talk to his troubled niece, Simone. Simone turns out to be more troubled than Maurice knows -- she's a prostitute and Kagan's accomplice. MacLeod goes after Kagan, who protests his innocence in the deaths of the guards and who promises MacLeod that he'll change his ways if MacLeod will help him. When Simone, the only one who knows Kagan is the killer, is killed, MacLeod realizes that Kagan could never really change and confronts Kagan, taking his head.