Buffy The Vampire Slayer (1997) - 4x16 - Who Are You? (2)

After Faith switches bodies with Buffy via the device that was left for her by the Mayor, she begins to slip into Buffy's life. When the police try to take away Faith (Buffy in Faith's body) she is kidnapped by a special group from the Watcher's Council. As Buffy tries in vain to convince them that she is not Faith, Faith in Buffy's body is out having a good time. Willow attempts to introduce Tara to Buffy but when Tara senses that something is amiss they perform a spell which uncovers the switch.
The man/demon/machine Adam has determined that his purpose is to kill and recruits some vampires to help. The vampires take over a church and both Faith and Buffy arrive to stop them. Using a Katra that Willow and Tara conjured, Buffy gets her own body back and Faith runs and disappears without a trace.