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 her sisters' fate is now known: Both Prue and Piper have lived, having been healed by an escaped Leo. Phoebe was also able to escape, but Cole has been either captured or on the run. The three sisters reunite and discuss the situation. It seems that even with the Power of Three, they do not have enough power to vanquish Shax. The sisters attempt a power enhancing spell, but with no luck. Or so it seems…..
The following morning, at the social services agency, a newspaper appears in the cubicle of assistant Paige Matthews. It prominently bears Dr. Griffiths' obituary, and Paige feels the need to leave.
At the manor, Leo, Cole and Darryl meet the surviving sisters. Cole is unwilling to go to the doctor's funeral, as the Source is still after him. Leo is sorry that he was unable to save the doctor and Piper feels guilty that he saved her instead. In the underworld, the Source consults the Oracle, and here he reveals 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. Demons attack Cole, and he and Piper fight them off. Piper, Cole and Phoebe agree to try and protect Paige from Shax, even though the Power of Three is not enough.
Prue, frustrated with the situation, summons Grams. Grams says that she and Patty had been discussing the situation themselves, but she still does not reveal to Piper the whole truth.
The Source again consults the Oracle, who shows him Paige dancing at P3. Paige, it turns out, has been coming to P3 for the last year. She tells her life 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. She and he leave the table, and it turns out that Paige has idly drawn a four sided triquetra on a napkin.
Cole, Phoebe and Piper 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 everyone's amazement, Paige briefly orbs, dodging Shax's attack. Cole, Phoebe and Piper shimmer to the rooftop and Cole throws energy balls while Paige flees. Piper and Phoebe say the vanquishing spell, repulsing Shax. Shane recovers in the hospital with only a concussion; Paige meets him and then Inspector Cortez talks to them, telling them of the doctor who was killed in a similar manner, also by Shax. Paige begins to suspect her connection to the Halliwells.
The Elders know nothing of Paige, and can't explain how she can orb. Cole reports that the Source thinks Paige is a witch and possibly even a Charmed One. Prue runs to the attic, with Piper, 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: after she and Victor divorced, she had an affair with her Whitelighter, Sam Wilder. They had a child together, and Grams had talked Patty into giving up the baby at a church. If they can bring Paige to the Book of Shadows, then the Charmed Ones might have enough power to vanquish Shax.
Cortez enters the attic with Darryl behind him. He finds Grams and Patty in ghost form, and accuses the girls of killing the doctor. At last, Darryl hits Cortez over the head and knocks him out. Phoebe urges Darryl to leave, and at Gram's insistence, Piper sends Cortez away -- to Timbuktu, because she needed a rhyme. Cole shimmers away to find Cortez.
After Grams and Patty leave, Leo, Prue, Piper and Phoebe come downstairs and find Paige in the manor. Prue shakes Paige's hand and a bright light comes over the four sisters. Shax barges through the door and the four sisters flee to the attic. Shax breaks the door and the four sisters recite the spell from the Book of Shadows, vanquishing Shax at last. Paige, however, is dismayed and runs from the manor.