With a "demon assault" having claimed the life of beautiful witch Prue Halliwell at the end of Charmed's third season, her sisters Piper (Holly Marie Combs) and Phoebe (Alyssa Milano) are left behind to mourn in their ancestral San Francisco mansion as season four begins. However, it isn't long before Prue's place is taken in the household (and the series) by the girls' half sister Paige Halliwell (Rose McGowan), who, like the late Prue, possesses telekinetic powers. It does not take long for Paige to contribute to the family coffers by taking a "mortal" job as a social worker, providing her with ample opportunity to activate her magical powers for the forces of good. Meanwhile, Phoebe's boyfriend, Cole (Julian McMahon), having successfully purged himself of his demonic alter ego, Belthazor, decides to leave town for a while to become accustomed to his newfound benevolent mortality. Cole returns just in time to rescue Piper and Paige from a powerful demon called The Source (Bennet Guillory). Though successful in this endeavor, Cole sacrifices his human form to The Source, who, in concert with manipulative female demon The Seer (Debbi Morgan), bamboozles Phoebe into marrying Cole's doppelganger. Before long, Phoebe is impregnated with the baby from hell. Season four ends as the Angel of Destiny (Dakin Matthews) appears before the Halliwell girls, offering them a demon-free life if they will vanquish The Source and its minions.