Piper wants to leave all of the sisters' possessions to Leo in case anything happens to them. Just then, Phoebe mentions she and Cole have found a way to vanquish the Source. Phoebe has created a spell that uses the magic of the entire Warren-Halliwell line against the Source. Leo orbs away to tell the Elders.
Phoebe isn't sure about Piper writing a will, but Piper doesn't want to leave anything to chance; Prue arranged to leave everything to them, and had she not done so, they'd still be tied up in probate court. Phoebe touches Cole and has a premonition of him being hit by a fireball to protect her. She tells Piper about it, but doesn't want to tell Cole, since the demon who threw it is one she hasn't seen before--one with only half a face. Since Cole is fully human now, she wants to keep him away until she can find out who it is.
Paige receives a woman, Carolyn, at social services. Carolyn is trying to gain custody of her son, but has made a lot of mistakes in her life and has recently gotten out of an abusive relationship. Paige and Darryl encourage her not to worry. Suddenly, Paige's phone rings--it's Piper, alerting her about Phoebe's premonition. Paige can't get out of the office yet. Meanwhile, Phoebe hustles Cole out the door.
The Guardians of the Hollow, one good and one evil, are playing chess, when the Source shows up. The Source vanquishes the good Guardian and removes the Hollow from its storage place. It is contained in a small black box. The evil Guardian tries to talk the Source out of unleashing the Hollow, but the Source opens the box and the Hollow enters the evil Guardian. The Source sends him to absorb powers and bring them back to him.
Piper and Phoebe are looking in the Book of Shadows, when the evil Guardian shimmers in. He throws an energy ball at Phoebe, sending her flying into a mirror. Piper tries to blast him, but the Guardian absorbs her power. Piper grabs an athame on the table and throws it at the Guardian, vanquishing him. During the scuffle, Phoebe is injured by a huge glass shard stuck in her back. She nearly dies, but Leo is able to heal her. Piper discovers that she's somehow lost her powers. She finds the Guardian in the Book. When Leo sees this, he's greatly alarmed. He tells Piper and Phoebe about the Hollow. It's a powerful force that absorbs all magic. The last time it was unleashed, good and evil had to join forces to contain it. Piper and Phoebe suspect that the Source was desperate enough to release it, and will likely send another demon after them. If that happens, their powers won't do any good, since that demon will certainly have the Hollow as well.
Carolyn hasn't shown up for her hearing, worrying Paige and Darryl. Paige fears Carolyn's boyfriend has tracked her down. Just then, Phoebe rings in to warn her about the attack from earlier. Paige heads for the manor and asks Darryl to stall for time.
Piper and Phoebe are making a potion, when Cole comes back. Phoebe finally tells Cole of her premonition and describes the demon she saw. Cole tells Piper and Phoebe that the demon is the Source.
In the Underworld, the Source is testing out Piper's powers. The Seer is concerned, but the Source is determined to get all of the Charmed Ones' powers. The Seer fears that the Hollow will eventually consume the Source, but the Source won't hear of it. He summons a Darklighter and puts the Hollow into him. The Source orders the Darklighter to kill Leo.
The sisters are readying for the Source's expected attack. They've set up a Crystal Cage, and Piper has made a potion to replicate her blasting power, while Phoebe has her vanquishing spell ready. Leo orbs in and confirms everyone's worst fears--the Source has unleashed the Hollow. He also says they need good and evil magic to contain it again and restore Piper's powers.Just then, Darryl calls-- Carolyn's boyfriend found her and gave her a savage beating. Darryl managed to delay the hearing for an hour.
The Darklighter attacks in the manor and shoots Leo with his crossbow. Paige tries to orb the crossbow, but the Hollow steals her power. The Darklighter targets Paige, but Phoebe vanquishes him with a potion. Piper pulls the arrow from Leo's stomach. She wants Leo to switch powers with Phoebe, so Phoebe can heal him, but Leo is afraid the Source could come back before then. Phoebe is worried; it seems as if the Source is one move ahead of them.
The Seer has a vision of everything being destroyed, unless the Source puts the Hollow back in the crypt, but the Source is determined to kill the sisters.
Cole gives Phoebe an engagement ring with a large diamond on it. Darryl brings Carolyn to the manor. Her face is covered with cuts and bruises. Paige casts a spell to heal Carolyn's face, and tells Darryl to get her to the judge and get her boyfriend off the streets.
Cole and Phoebe have an idea to get Piper and Paige's powers back--use the To Call a Witch's Power spell to get them back from the Hollow. Cole goes down to watch Leo, but the Seer casts a spell and Cole finds himself in the Underworld with her. She greets him as Belthazor, and proposes to help him and the sisters defeat the Source. The Seer tells Cole that if he takes in the Hollow, he can absorb the Source's powers, enabling the sisters to vanquish him. Once the Source is dead, the Seer then intends to help the sisters contain the Hollow again. Cole is suspicious, but the Seer tells him this is the only way to stop the Hollow from destroying everything. Cole agrees and the Seer puts the Hollow into him.
The sisters are casting the To Call a Witch's Power spell, when The Source attacks in the manor. He blasts Piper with a fireball, knocking her out. Phoebe and Paige carry Piper to the attic and set up the crystal cage, but he merely orbs one of the crystals. The Source throws a fireball at Phoebe, but Cole shimmers in and runs in front of the fireball, thus absorbing the Source's power. Cole throws fireball after fireball at the Source, distracting him long enough for the sisters to vanquish him with Phoebe's spell. The Seer appears a moment later and she and Phoebe read an inscription on the Hollow box. It is, thus, restored to its proper place and Piper and Paige have their powers back. Phoebe asks, at one point, where the Source's powers went and the Seer cryptically replies, "into the void". Paige can now heal Leo.
As the sisters, Leo and Cole celebrate at P3, we see Cole's eyes turn black, implying that he is now the Source.