Episode 15 of "Angel"
s01e15 The Prodigal

Angel has a memory of an altercation between him and his father in 1753, but his mind quickly returns to the present when he kills a demon who escaped from a moving subway train. When Kate enters on the scene, he tells her to cover up the truth to her fellow officers. Though she is disturbed by the demon factor, she becomes distracted after seeing her father enter the station. Later that morning, Wesley identifies the deceased demon as a Kwaini species and remarks it is atypical for this type of demon to be violent. They deduce the demon had an incentive to attack one of the passengers. Angel then tries to obtain a list of passengers from Kate, but she's unwilling to give it to him. Despite this, he manages to locate the passenger in question and follows him on one of his deliveries: to Mr. Lockley's apartment. When Angel confronts Mr. Lockley, he denies any involvement. Angel, reminded of his painful past with his father, has another memory from his past: he meets Darla and is sired. Back at A.I., Wesley conducts some tests and discovers the Kwaini demon was addicted to a substance similar to street PCP. Angel deduces that Kate's father is somehow involved with the drug trafficking. He then has another memory, this time of his own funeral, his first meeting with Darla as a vampire, and his first kill. That afternoon, Kate approaches Angel, gives him a list of the passengers and tells him she wants to be involved with the case. She changed her mind after having a conversation with her father. Suddenly, two Kwaini demons attack Wesley and Cordelia, seeking more of the drug. While this is happening, the drug traffickers pay a visit to Mr. Lockley. Meanwhile, Angel has another memory of killing his entire family. Angel tries to save Mr. Lockley from the traffickers but can't enter his apartment because he was not invited. Standing by the door entrance, he watches the vampires kill Mr. Lockley. After his death, Angel enters in and kills one of them. Kate arrives and is devastated by the death. She obtains the address of the traffickers and manages to slay one of the killers. As the rest of the gang prepares for attack, Angel bursts in and slays the lot of them. He has one final memory from his past: Darla informs her that he will carry the painful memories of his father for a lifetime.