Paige is with Darryl. A young single mother lost custody of her child due to a past history of abuse by her ex-husband. They are trying to help her through the court hearing to get her son back. They are waiting for the verdict when Paige gets a call from her sisters. They have been attacked and she needs to come home. Carolyn is currently missing, so they postpone the legal proceedings and Paige heads home.
Darryl calls Paige and lets her know that they found Carolyn. She is bruised and swollen. They suspect the ex-husband, but it is doubtful that they will find him.
Darryl brings Carolyn to the manor. Paige grabs the Book of Shadows and Piper warns her that she can't do this because it's personal. Considering the dire situation around them regarding the Source, she doesn't care. Carolyn is an innocent and she's going to help her. She whispers a spell and all of Carolyn's bruises disappear. She tells Darryl to take care of Carolyn and see that her ex ends up behind bars. He is worried. He senses that something is terribly wrong. He has to go, and escorts Carolyn out.
The Source is back. The Seer looks into the future and sees the Source's demise at the hands of the Charmed Ones. She tells him that the Halliwells are too powerful for him. Unable to give in to defeat, the Source decides to change his future. He plans to obtain the Hollow. The Seer warns him that it is forbidden, and dangerous for all.
Phoebe finally figures out what spell they need to defeat the Source. The power of three alone, is not enough. They need to pull from the power of all their ancestors. When she goes to tell Piper and Leo, they are discussing Piper's will. Prue had taken such precautions and saved them all a lot of hassle. Leo doesn't like talking about it, but she wants to make sure everything is taken care of in the event of her death. Phoebe interrupts to tell them of the new spell. After Leo leaves to tell the elders, Phoebe has a premonition that Cole dies, while saving her from a demon with half a face. She sends Cole out, so she and her sisters can figure out what to do.
The source retrieves the Hollow. It is a force, neither good nor bad, that can absorb powers. Not just of good witches, but evil sources as well. He sends a demon with the hollow to fight the Charmed One's. The hollow will take the powers from the sisters, so the Source can transfer them to him.
The demon attacks Phoebe and Piper at the manor. Phoebe is severely wounded. Piper tries to blow the demon up with her power, but instead her powers are drawn from her into the hollow. She sees an Athame and manages to throw it at the demon vanquishing him. She sees the hollow (a black mass) rise from the demon as he dies. Leo comes to heal Phoebe. Her wounds were severe, and she almost died.
They search the Book of Shadows and find the description of the Hollow. It is so dangerous that both evil and good had to ban together to contain it. It could not be destroyed. An angel and a demon, sworn to keep it safe, were guarding the hollow. That is, until the Source broke the alliance and stole it for his own purposes.
They call Paige to come home. Piper starts working on a potion to simulate her missing powers. She doesn't want to be defenseless against this new evil. Cole comes home and Phoebe tells him what's going on, including her premonition. He tells her that the demon with half a face is actually the Source.
The Source now has Piper's powers. The Seer warns him again of the dangers of what he's doing. The consequences are sure to be grave for everyone. He ignores her concerns. He realizes that he needs to get the sister's Whitelighter out of the way. He sends a Darklighter infected with the Hollow.
Back at the manor, Piper finishes a potion that will simulate her exploding power. Leo orbs in to report that he's confirmed that the Source has unleashed the Hollow. The Darklighter orbs in right behind him. Paige cries out to him, but it is too late. The Darklighter plunges his arrow into Leo's side. He prepares to shoot Leo again, when Paige calls for the arrow. Instead, the Hollow absorbs her power. Phoebe throws a vial of Piper's potion and the Darklighter explodes.
Piper is cradling the mortally wounded Leo. She pulls the arrow out. There is no antidote to the Darklighter's poison in the BOS. Paige's powers are gone, so she can't use her them to help. For now, there is nothing they can do.
The Source is pleased to have another power. He asks the Seer what the future holds now. She sees nothing, no magic, no life, and no world. He doesn't care. She pleads with him again, but it does no good. He leaves the Hollow with her to guard, as he leaves to find another demon to gain the last power.
A devastated Piper tends to Leo. Cole goes to Phoebe and hands her a box. It is an engagement ring. She try's to say this isn't the time, but he gives it to her anyway. No matter what happens, he wants her to know that he loves her. She try's to get him to leave, fearing that her premonition might come true, but he won't. If it's his fate to die saving her, then it's worth it.
The girls are going to try a spell. They are hoping they can call for their powers and draw it from the Hollow. They leave Cole with the Leo. Suddenly, Cole is pulled to the underworld by the Seer. He is confused, but she explains she had a vision about him and her doing great things together. He refuses, but she tells him he should reconsider if he hopes to save Phoebe. She tells him of her plan and he agrees. She opens the Hollow box and it surges into him.
The sisters try their spell but instead of it pulling their powers from the Hollow, it brings the Source to them. Their powers are now his. The girls try to get away, but the Source hurls a fireball at Piper, sending her flying into a wall. Phoebe and Paige drag Piper upstairs. They try to enclose the Source into a force field using a ring of crystals. But he uses Paige's power to call one of the crystals, breaking the force field. He hurls a fireball at Phoebe, but before it hits her Cole appears and jumps in front of it. Instead of killing him, he absorbs the power, and Phoebe realizes he has the Hollow. The sisters take the opportunity and chant their spell, using the entire Halliwell witch line for power. The Source is vanquished. The Seer enters and removes the Hallow from Cole. She turns to Phoebe and the two of them recite an inscription that will contain the Hallow once again. This returns the powers the Hallow has consumed.
Paige goes to Leo, barely hanging on. She holds his hands and draws from his power to heal him. The Seer must go, but before she does she mentions that it's a shame Belthazor is gone. "He would have made a great new Source." The girls wonder what happened to the Source's power when theirs were returned to them. The Seer mentions that it has gone into the void. The girls don't understand, but the Seer's comment gives a clue that the Source's powers have been transferred to Cole. She disappears and Piper asks who she was. Cole says she's evil, but Phoebe says she saved their lives and helped them kill the Source.
At P3, they celebrate. They wonder if vanquishing the Source means they can relax, that the nightmare is finally over. They toast to Prue. Piper and Leo head home. Paige goes to talk with a cute guy. Phoebe and Cole are left alone. She is ready to celebrate and focus on their future together. She tells him that he's her hero. He tells her that he's no one's hero, and unbeknownst to her, his eyes turn black ominously.