Through a Glass, Darkly - S4.E18

MacLeod's old friend Warren Cochrane is hiding a horrible secret he can't bear to remember. Realizing that an Immortal who won't remember what he is is soon a dead Immortal, MacLeod tries to help Warren by reminding him of the history they shared together, of their battles for Scotland's freedom, and of their mission to return Bonnie Prince Charlie to the throne. But MacLeod might have helped his friend more by letting the past stay buried.