Paige has pawned Billie off on Leo for the day. Leo is trying to teach Billie the history of good magic, but Billie isn't interested and prefers to swing her nunchaku. She does sound more interested, however, when Leo mentions Magic School. Billie starts telekinetically flinging her nunchaku and accidentally propels it out the attic window.
Piper is making a costume for Wyatt's preschool play. Paige comes down and suspects Wyatt orbed some of her jewellery out of her room. She finds Wyatt playing with it like a toy. Paige is on her way to a coffee date with a guy she met on an online dating service. She isn't sure what lipstick to wear, but Piper suggests one that matches the face of "Jo." Paige is worried about Phoebe; she suspects Dex sees other women. Phoebe comes in; she's done several magical calculations and is convinced Dex is "the one."
Piper takes Wyatt to preschool and leaves him with his room mom, Mandi. Wyatt is having trouble with his costume, but Mandi finds a pin in it. She reminds Piper of another costume that Wyatt needs - a pumpkin. Mandi takes Wyatt with her, ostensibly to prepare for rehearsal. Once she's alone, she sends a part of Wyatt's costume up in flames, and her eyes turn black.
Paige goes to her coffee date. She describes herself as a former special needs teacher who now tutors one-on-one with a student who's a real pain at times. Paige's date becomes rather too talkative and Paige, bored, sees Dex at another table giving a kiss and a hug to an unknown woman.
Phoebe is setting up a photo shoot for "Julie" when Paige comes in. Paige wonders how well Phoebe really knows Dex. Phoebe gets suspicious, reminding Paige that they promised to stay out of each other's love lives. Paige thinks she and Dex are going too fast, but Phoebe feels she has to honor her premonition about him.
Billie is still telekinetically swinging her nunchaku, but stops when Piper and Leo walk into the attic. Piper wants to use magic to make a good pumpkin costume for Wyatt, but Leo thinks it crosses the line into personal gain. Billie pretends to be studying, but wants to go to Magic School. Leo isn't willing, fearing it might expose himself and the sisters. Piper claims she doesn't want to see Wyatt humiliated, but Leo suspects it's more about Piper not wanting to be humiliated by Mandi. Piper finally relents, but still vows to make a costume that will have Mandi eating crow.
Magic School is now controlled by demons. Mandi shimmers in, and plans to bring Wyatt back there once she can get him to trust her enough to use his powers around her. The demons were supposed to have the place already cleaned up, and Mandi isn't pleased. One demon isn't moving fast enough for her, and Mandi chokes him to death. Suddenly, Mandi collapses, and a hideous Female Possessor Demon comes out of her.
Piper has been up all night making Wyatt's pumpkin costume. She's worried about Wyatt, though. Even though he can see her as Piper, everyone else sees her as "Jamie," Piper's cousin. Paige wants to find out who was kissing Dex earlier. Piper is skeptical, and suggests she might be Dex' sister, but Paige isn't buying it.
The possessor demon walks over to a demon painting a pentagram on the wall. He wants to know who they're bringing back. The possessor demon is reticent with details, even though the other demon is the only one who can bring dead demons back from the the Demonic Wasteland. He also doesn't understand why they need Wyatt. The possessor demon points out that he's the only one with enough power to bring back a very powerful demon from the dead. The other demon thinks it's far too risky to kidnap Wyatt, seeing that he's the son of a Charmed One. The possessor demon then reveals that they're bringing back the Source of All Evil and she intends to turn Wyatt evil and raise him as their own son. She then reenters Mandi.
Paige finds the woman whom Dex kissed and talks to her about Dex. The woman gives her name as Sylvia. She says that she had broken up with Dex months ago, but mentions "breakup sex."
"Mandi" tells Piper that she has to make another costume--a mouse this time. Piper is somewhat annoyed. "Mandi" thinks Piper is more concerned about her reputation than Wyatt's, and two of the other room moms back her up. Leo calls and mentions that Billie insists on going to Magic School. Piper thinks it's too dangerous. Piper then mentions Mandi: she thinks she's out to get her. She's also annoyed at how perfect Mandi seems. Leo comes back to the attic and says Billie can't go. However, Billie is insistent. She throws a teleportation potion, sending both herself and Leo to Magic School. They find it overrun by demons, and Billie didn't think to make one to get them back home. Leo remembers a spell in a book and they try to find it, when two demons spot them. Billie flings a nunchaku and knocks them out; the two dress in the demons' clothes.
Piper comes home expecting to find Leo with Wyatt, but no one is there.
"Mandi" gains Wyatt's trust by conjuring a cookie and offering it to him, then convinces him to orb a sandwich Piper made for him into a trash can.
Paige glamours into Sylvia and enters Dex's studio. She says she wants no more breakup sex and that he needs to be true to Phoebe. Dex, confused, says he hasn't slept with Sylvia in two months. Phoebe walks in and "Sylvia" blurts her real name; she covers, at least for Dex, but Phoebe is alerted. An embarrassed Paige hustles out and drops her glamour.
Leo and Billie are trying to figure out how to get home. Leo notices everything is all out of order. Suddenly, "Mandi" fades in with Wyatt. "Mandi" has Wyatt play with some toys while Leo continues to look for the book. The demon working on the pentagram thinks the Source isn't going to be happy, especially since it was thought not even the sisters could vanquish him. However, "Mandi" thinks that the Source will be grateful to her for bringing him back and giving him a son as well. Leo overhears this and is visibly alarmed. Billie thinks she should stay, since she has powers. However, Leo doesn't think Billie is powerful enough to take on the Source, and is determined to save Wyatt even without powers. He tells her to use the spell to go back home herself.
Piper goes to the preschool and finds Wyatt missing. He is signed out with Mandi. The teacher doesn't think there's a problem at first until Piper notices that Mandi's own son is still there. Piper suspects a demon is involved and storms out.
Paige comes home and tries to sneak upstairs, but Phoebe tackles her. Paige admits she posed as Sylvia. Just as they start fighting, Piper walks in and reports that Wyatt's been kidnapped and that Leo's missing. Piper is kicking herself; she realizes that a demon somehow possessed Mandi and used her to get at Wyatt, and "Mandi's" all-too-perfect facade was a classic sign of demonic possession. Paige scries while Phoebe starts working on a dispossessing spell and a vanquishing potion. Just then, Billie walks in and reports that Wyatt is at Magic School, which is now controlled by demons. She also mentions that they wanted Wyatt to help bring back the Source. She wants to help vanquish him, but the sisters, like Leo, think she isn't ready.
"Mandi" speaks a spell and has Wyatt place his finger in a chalice to seal it. A portal forms on the pentagram, and the Source tumbles through. "Mandi" proposes to the Source that they raise Wyatt as their own. The Source is skeptical at first, but "Mandi" tells him that he's the son of a Charmed One and that his mother and sisters are dead. Leo speaks to them and Wyatt orbs to him. Leo claims he's a fellow demon, but the Source is suspicious.
The sisters orb in just as the Source recognizes Leo as Wyatt's father. Just as the Source is about to kill Leo, the sisters speak a spell offscreen and eject the possessor demon from Mandi. The sisters come out of hiding. Before the Source can react, however, Piper blows up the possessor demon. The Source is sucked back into the Wasteland; since the possessor demon conjured him, they were magically connected.
Billie sounds more serious about studying the history of good magic. Dex comes over to ask Phoebe's forgiveness for "Sylvia" popping in causing Phoebe to thank Paige for meddling this time.
Piper and Leo go to Wyatt's play. They spot Mandi, and wonder if she has any idea what happened.