Charmed: Sleuthing With the Enemy (2000)

Synopsis
Now aware that Cole (Julian McMahon) is actually the demon Belthazor -- or rather, the "Belthazor" side of his personality is stronger than his "human" side -- Prue (Shannen Doherty) and Piper (Alyssa Milano) join forces with bounty hunter Krell (Scott McDonald), determined to destroy Cole before he destroys them. Meanwhile, Phoebe (Holly Marie Combs), still unaware of Cole's dual personality, tries to find him after he disappears from view. The climax of the episode finds Phoebe taking drastic measures to save Cole from her relentless sisters. ~ Hal Erickson, All Movie Guide