Spike returns to Sunnydale alone in Season Four, in the episode "The Harsh Light of Day", briefly living with Harmony Kendall, a shallow former classmate of Buffy, now a vampire [REF].
After being captured by The Initiative and implanted with a cerebral microchip which punishes him with debilitating pain whenever he harms or attempts to harm any non-demon life forms, Spike turns to the Scooby Gang for protection, bartering his knowledge of the Initiative.