At the end of Season 3, the fate of the Charmed Ones was uncertain, with Phoebe in the Underworld and Prue and Piper gravely injured. How Phoebe returned to the mortal world is not immediately revealed, but Cole apparently helped her escape eventually and her sisters' fate is now known: Piper lives, healed by Leo, but Prue is dead as Leo apparently didn't return in time to save her.
Piper, grieving, tries one spell after another to summon her dead sister, but in vain. She and Phoebe have tried numerous ways to get Prue back over the last week, but to no avail. After Phoebe convinces Piper to get some rest, the triquetra on the Book of Shadows glows red, and the book opens to the last spell Piper tried, "To Call a Lost Witch".
The following morning, at South Bay Social Services, assistant Paige Matthews is at work when a candle at her desk mysteriously blows out. Suddenly, lights swirl around her head, and a newspaper drops out of thin air behind her. It prominently bears Prue's obituary, and Paige feels the need to leave.
At the manor, Victor, Leo, Cole and Darryl meet the surviving sisters. Cole is unwilling to go to Prue's funeral, as the Source is now after him with a vengeance. Leo is sorry that he was unable to save Prue, and Piper feels guilty that he saved her instead.
In the Underworld, the Source consults his Oracle, and here he reveals that Phoebe was saved by Cole and Leo. The Oracle tells the Source of another possible Charmed One.
The funeral is done, and Paige is among the attendees. Paige approaches Phoebe, perhaps not knowing why, and Phoebe falls over from her premonition: Shax is after Paige, and she sees him attack on a rooftop with a helipad. At this point, Demonic Bounty Hunters attack Cole, but he and the sisters fight them off. Piper is visibly angry; she can't believe the demons didn't have the decency to leave them alone at least this once. Phoebe and Cole agree to try to protect Paige from Shax, though they don't have the Power of Three.
While Darryl's partner, Inspector Cortez, arrives to inform the Halliwells he's on the case, Piper, frustrated, tries to summon Prue with a spell, but her spell summons Grams instead. Grams says she couldn't come earlier because she and Patty were with Prue. Grams tells Piper that she can't see Prue just yet, that she must let go.
The Source again consults the Oracle, who shows him Paige dancing at P3. Paige, it turns out Paige has come to P3 for the last year. She tells her story to her date, Shane: that she was adopted, that her adoptive parents had died, and she is looking for her birth family. She thinks she might be related to the Halliwells, though she is uncertain about it and unwilling to approach them. Both leave the table, and it turns out that Paige has idly drawn a triquetra on a napkin.
Cole and Phoebe stake out a rooftop that Phoebe believes to be the right one. Sure enough, Paige and Shane come onto the rooftop and embrace. Shax appears and blasts Shane, and then turns on Paige -- and, to everybody's amazement, Paige briefly orbs, dodging Shax's attack. Paige runs downstairs, but Shax chases her down. Cole and Phoebe shimmer in. Cole hits Shax with energy balls, and Phoebe sends him away with the vanquishing spell. Shane recovers in the hospital with only a concussion; Paige meets him and Inspector Cortez talks to them, telling them they're lucky to be alive because their attacker was the same one that killed Prue and Dr. Griffiths. Paige begins anew to suspect her connection to the Halliwells.
Leo reports that the Elders know nothing of Paige, and can't explain how she can orb. Cole reports that the Source thinks Paige is not only a witch, but related to Prue, Piper and Phoebe--a fourth Charmed One. He thought the Power of Three had died with Prue--hence his decision to send Shax after Paige. Piper runs to the attic, with Phoebe, Leo and Cole after her, and again summons a puzzled Grams. Grams says she is sworn to secrecy, and won't answer, but Patty then appears, and explains Paige's origin. Paige is her youngest daughter, the product of the relationship between Patty and her Whitelighter, Sam Wilder. Although Patty had initially wanted to keep her, she and Grams ultimately decided to give her up out of fear that the Elders would strip Prue, Piper and Phoebe's powers as punishment. If they can bring Paige to the Book of Shadows, then the Charmed Ones might be reconstituted.
Cortez enters the attic with Darryl behind him. He finds Grams and Patty in ghost form, and accuses Phoebe and Piper of practicing evil magic as well as killing Prue and the doctor. After hearing him threaten to put them under constant surveillance, Darryl hits Cortez over the head and knocks him out. Phoebe urges Darryl to leave, and at Grams's insistence, send Cortez away -- to Timbuktu, because she needed a rhyme for 'undo'. Cole shimmers away to find Cortez.
After Grams and Patty leave, Leo, Piper and Phoebe come downstairs and find Paige in the manor. Piper shakes Paige's hand. The chandelier shimmers, and bathes the three sisters in a bright light. Shax barges through the door and the three sisters flee to the attic. Shax breaks the door and the three sisters recite the spell from the Book of Shadows, vanquishing Shax at last. Paige, however, is frightened and runs from the manor.