Every superhero has to lose their powers at some point for some reason. Buffy the Vampire Slayer made the most of the tradition in "Helpless", a season three episode from 1999. The second episode to be written by David Fury, he makes good dramatic material from the reason for Buffy's power loss while director James A. Contner takes the opportunity to make a more traditional horror story of the kind Buffy is usually a subversion of.
Buffy (Sarah Michelle-Gellar) almost gets killed during a routine vampire fight, embarrassingly almost being stabbed with her own stake. During the day, Giles (Anthony Stewart Head) has been giving her some kind of strange crystal focus training.
He's already acting suspicious while he's doing it but it's not until around one third of the way through the episode that we find out it's a combination of his hypnosis and drugs that are causing Buffy to lose her powers. He does it at a particularly potent time for dramatic purposes as she's heavily hinting she wants him to take her to an ice show in place of her own absent father. But Giles was commanded to suppress Buffy's powers by his superior at the Watchers' Council, played by Harris Yulin, perhaps best known for playing the judge in Ghostbusters 2.
I love the herringbone tweed sport coat with the extra straps.
He's sinister and authoritative enough you forgive his feeble attempt at an English accent, though it makes it puzzling that of his two English assistants, the only one played by an English actor barely has any lines--especially puzzling since it's Dominic Keating, who went on to be part of the main cast of Star Trek: Enterprise.
He does become a vampire, though, a disciple of the very creepy, Cape Fear-ish serial killer vampire Kralik (Jeff Kober). He stalks Buffy through the creepy old house where he's kidnapped her mother and apparently taken numerous photos of her.
They put Buffy in some evocatively vulnerable, juvenile outfits in this episode.
Kralik mentions an unfortunate past with his own mother, implying she castrated him with a pair of scissors, altogether making an episode that doesn't paint the rosiest picture of parental figures. Yet the internal conflict Giles feels as the institution designed to help the Slayer compels him to betray her trust is one of the best story points ever for Giles.
It's great because you can see both Giles' and Buffy's points of view even as her feelings of disgust and betrayal are completely reasonable. Which makes it all the more effective when she softens a little at the end. She gets it, it was the judge from Ghostbusters 2, but everyone would hope our loved ones would do a better job using their own judgement. It's a nice moment in the season arc, too, about rebellious youth and the uncertain reliability of authority figures.
Buffy the Vampire Slayer is available in a lousy cropped format on Amazon Prime.