Their secret affair is exposed when Oz and Cordelia catch them kissing while they are held captive by Spike.
Oz, having smelled Willow's fear and tracked her scent, witnesses the betrayal. He subsequently refuses to speak to her. Watch Btvs 3x8 mini
"Lovers Walk" (Season 3, Episode 8) is widely considered one of the most pivotal episodes of Buffy the Vampire Slayer (BtVS), as it permanently reshapes every major romantic relationship in the series. Episode Title: "Lovers Walk" Original Air Date: November 24, 1998 Director: David Semel Writer: Dan Vebber Their secret affair is exposed when Oz and