Piper is preparing for Henry and Paige's engagement party and Phoebe is helping Billie take Christy out into public after her years of imprisonment. Meanwhile a demon, Zar, is sending demons to take out the Triad, and he considers tricking them into going after the Triad.
Paige is going through the Book of Shadows and isn't apparently very concerned about matters, but Piper suspects cold feet. Paige reveals they've been fighting and Piper advises her to listen a bit. Zar reveals the Triad have taken over the Magic School and the Triad banished him. Christy's public dining isn't going well and she inadvertently sets off a fire with her powers. As Phoebe heads back she bumps into an attractive guy but doesn't even notice, much to Billie's consternation.
At the manor Billie is trying to show Christy how to toast without much success. Two policeman show up to check up on Piper and ask if she's anything suspicious. They're Zar's demon henchmen and they try to warn her about beings they've dealt with but haven't without mentioning the Triad. Christy "senses" them and a fight breaks out – Christy incinerates one and the other disappears.
Everyone ponders why demons would try to warn them and Piper insists the party is still on and sends the stalling Paige to talk to Henry. Zar is not happy with his assistant's failure but is happier when he reveals that Christy was present. Zar wonders if she is "the One" and how the Triad was planning on acquiring the "Ultimate Power."
Paige catches up to Henry for ring-shopping just as the store closes. He initially understands when she explains the demons, but they're soon fighting again. At Phoebe's apartment, the cute guy from earlier shows up and comes on to Phoebe again. She's suspicious of the guy but they're interrupted when Zar grabs Christy and teleports out.
The sisters compare notes and as per Zar's plan, figure the Triad took her. They figure this is the big battle that will let them get Leo back, while Phoebe continues to suspect the neighbor as one of their lackeys. Phoebe goes to the guy's supposed apartment but it's empty. She goes to her apartment, where he's waiting. After slugging him, he freezes her using a ring and tells her about everything she's losing by rejecting love. Then he unfreezes her and talks some more about her not finding love, and reveals he's one of the Cupids – "Coop." The Elders sent him because of how everything has messed up her love life, but she'd rather deal with demons.
(from what i saw, cupid walks in on Phoebe earlier, and has the key that unlocks her apartment, he tells her he lives in the apartment under her. Later, Phoebe goes to her apartment and finds it completely empty, then she goes down to Cupid's apartment.)
Paige begs off with Henry to deal with the Triad and he's not happy. They've had no luck finding a banish for the Triad and Paige wonders why Christy's kidnapper didn't attack them. Billie's working on an enhancement potion out of desperation but not having luck, while Zar interrogates Christy and she manages to burst free, incinerate Zar's assistants, and send him flying. She disappears before she can do anything, when Billie's spell brings her back to the attic. While Paige goes to get Phoebe, Christy wanders off downstairs, senses demons, and the Triad appears. Christy clearly knows who they are and informs them that someone is impersonating them.
The Triad don't have the power yet to attack the Charmed Ones, but they figure it's Zar and it becomes clear they need Billie. Paige and Phoebe return and Phoebe tells them about Coop. Paige wants to meet with Henry, overriding Piper (who wants her to stay), while the Triad confront Zar in his lair. He taunts them over their lack of power and they incinerate him. They then discuss their plan and determine they must kill or be killed, despite the fact that currently they are evenly matched.
Paige meets with Henry and warns this may be their final fight, and that she can't have Henry along to help them. He vows to marry as soon as they get back. Phoebe runs into Coop back at her apartment and he wishes her luck in the battle with the Triad. Back at the manor they track them to Magic School and plan to divide them up, have Piper freeze them temporarily, and Paige then orbs one into the proper position. They head out, leaving Christy and Billie behind.
The Triad are in the middle of some ritual when the sisters arrive. Phoebe tries to distract them and they attack her – one of them runs off, Paige gets them to shoot at her before orbing out, and Piper freezes them. Paige orbs Asmodeus into the path of the other's fireball, destroying him, and Piper manages to take out Asmodeus's killer. The third one returns and then teleports out. Piper wonders why if they've won, Leo hasn't returned, but Phoebe wonders if there's a bigger threat out there. They return to the manor and start the engagement party, and Paige wants it to be a wedding.
But then everyone freezes and the remaining Triad member appears and speaks with the unfrozen Christy. He tells her they've shown her the way and she must go forward with her destiny and unite with the ultimate power…Billie. He leaves and everyone unfreezes with none the wiser. The next day they get ready for the wedding and after everyone is ready, the ceremony begins. They recite their vows and are happily married.