Lies My Parents Told Me
New York City, 1977: It's pitch black and pouring rain when the Slayer takes on Spike, while her young son looks on from behind a bench. The fight ends in a draw, and the Slayer, Nikki, explains to her son Robin that even though he wants to stay with her, he can't. Her mission is to kill Vampires, and she must fight alone. In the present, the boy has taken on his mother's duties--killing Vamps--this time at the Slayer's side. Wood hates that Spike is also fighting at their side, but as of now, there's nothing he can do about it.
Later, Giles and Wood are introduced, and Buffy gives the principal a quick rundown of their history with Spike, his soul, the chip and the trigger implanted by the First. Giles is concerned that the trigger is still active, and he's brought something home to help. It's a Prokaryote stone, to be implanted in Spike's brain (with Willow's magical help). The stone is sort of like therapy: it can root out the nature of the trigger by unlocking his subconscious. When it's implanted, Spike is transported back to...
London, 1880: William recites some sappy poetry to his mother, Anne, who tenderly approves of his talent. He loves her very much, but he worries over her recent illness, which looks suspiciously like tuberculosis. After her violent coughing fit passes, Spike sits at his mother's feet as she sings a familiar tune. The tune is what the First chose as the trigger.
In the present, Spike turns, lashing out and throwing furniture around. Giles is certain there's something more there that he's blocking, and Spike drifts back again, remembering his rebirth as a Vampire. He and Drusilla visit his home, and Dru is shocked to find that William wants to make his mother into a Vampire, so they can be together forever. "It only hurts for a moment," he whispers to Anne, as he bites into his beloved mother's neck.
Wood takes advantage of a moment alone with the former Watcher, and convinces him that everyone would be better off with Spike dead. He needs Giles to get Buffy out of the house for a while, and Giles agrees. (Note to Giles: this is probably a bad idea.)
Robin lures Spike to his "workroom," where the walls are covered with crosses. He claims to be about to help uncover Spike's trigger, but the Vamp realizes something's going on when Wood reveals that Spike killed his mother. Robin pulls up an mp3 of the trigger song, and against his will, Spike turns.
He relives first seeing his mother as a Vampire, glowing with health and vitality. But something’s changed. She's no longer the doting mother, but a bitter woman who wondered how long it would take her wimp of a son to get himself married and leave her alone. William is horrified, especially when his mother comes closer and attempts to kiss him, telling him, "You wanted your hands on me; perhaps you'd like to finish what you started?" Devastated, Spike shoves her away, and a struggle ensues. "I'm sorry," he whispers, and stakes her. For a moment, the true image of his mother looks upon her son gratefully, just before she turns to dust.
Meanwhile, Giles is trying to convince Buffy that difficult decisions must be made in the coming battle, but Buff doesn't get it. She's been making tough choices all over the place, but he remains adamant that there's more to be done. He's priming her to accept allowing Wood to kill Spike, but it's not until later that she realizes what's going on. Immediately she takes off to save Spike.
During the Vamp's visions, Wood's been beating Spike, but the moment his memory ends, the real Spike is back. Wood's no match for his strength and agility, and it's a rude awakening for the principal. Spike is matter-of-fact about killing Nikki: they were natural enemies, and it's not Spike's fault that eventually (however inadvertently) she abandoned her son. She was the center of Robin's world, but the center of Nikki's world was slaying Vampires, not being a mother. Wood is bitter, but Spike thanks him for clearing up the trigger. He's made his own realization: that his mother's soul had been taken over by a demon, that she truly loved him before she turned. He tells Wood he appreciates the help, then Vamps out, saying, "Just wanted you to know that, before I kill you."
When Buffy arrives at Wood's home, she finds Spike leaving the workroom. He spared Wood, but warns, "he looks at me funny again, I'll kill him." To an injured Robin, Buffy explains that they need Spike, and if something like this happens again, she'll let Spike kill him. "The mission is what matters," she says, echoing his mother's attitude.
Giles goes to Buffy, trying to convince her that killing Spike was for the greater good. He's still preaching when she replies, "I think you've taught me everything I need to know," and closes the door in his face.
Summary from Buffy.com