Dead on Arrival - S1.E22

A cyberhacker and his employer kill each other in a shootout...and the employer, an Immortal, walks away. The hacker's sister Janet is a friend of Amanda's and the duo get involved. Nick tracks a back-up CD to the Immortal, whom Amanda recognizes as Payton, an astrologer, poisoner, and stage magician who ran afoul of Amanda centuries earlier. Payton assaults Nick and then escapes with a poisoned-smoke grenade, giving the detective 22 hours to live. He'll give Nick the antidote if they stay out of his affairs. Amanda gets hold of the 10 million Payton was stealing, and Payton kidnaps Janet in return. They arrange to make a deal, and Amanda engages Payton in combat. He tries to trick her using a holographic projector but a dying Nick destroys it, giving Amanda the advantage. She suffers a rather bizarre Quickening as she flashes back over her life, and her encounters with Nick. Going to him, she asks for forgiveness and shoots him dead before the poison kills him. He wakes up immortal.