The episode where Buffy loses her Slayer powers is “Helpless.” It is the 12th episode of Buffy the Vampire Slayer’s third season, first broadcast on January 19, 1999.
The Watchers’ Council secretly tests Buffy on her 18th birthday by injecting her with muscle relaxants and adrenaline suppressors. The drugs remove her enhanced strength, speed, and reflexes, forcing her to confront the vampire Zachary Kralik without her usual abilities.
The test is especially painful because Giles has been ordered to participate and initially hides the truth from Buffy. After learning what happened, Buffy is furious with the Council, but she still completes the test by outthinking Kralik rather than overpowering him. She fills his medication glass with holy water, turning his addiction into the means of his destruction.
“Helpless” is also important for Giles’s character. Because he cares for Buffy like a daughter and helps her despite the Council’s orders, Quentin Travers fires him as her official Watcher. The episode is sometimes confused with stories in which Buffy temporarily loses her powers through magic, but this loss is caused by the Council’s deliberate experiment.