While Phoebe and Paige are having a dinner party with Jason and Richard, Piper is in a catacomb fighting off swarm demons. Leo tells Phoebe and Paige that Piper needs their help. While Richard keeps Jason occupied, the sisters try to fight off the demons. But as soon as they vanquish one, two more appear in its place.
Deciding that they need a new strategy, they orb back to the manor, but Jason walks in and sees them magically appear out of nowhere. He faints.
After he recovers, Phoebe tries to explain, but he drives off. Richard blames himself for what happened, saying he's carrying generations of his family's bad karma. Paige tells him that if anyone's bad karma is responsible, it's Phoebe's. She's been with Jason for more than a year, leading a double life.
Richard isn't convinced. He casts a karma cleansing spell, but it affects Phoebe instead. Unknown to the sisters, she's infected by the karma of Mata Hari, the legendary Dutch double agent who worked as an exotic dancer in France during World War I.
Piper and Paige learn that the swarm demons, like vampires and Kazis, can only be eliminated by defeating their leader. They locate the Swarm King in the catacombs, but they need Phoebe for the vanquish. They go home to find Phoebe scantily clad and draped with scarves, her speech interspersed with French phrases. She's not interested in helping them with the Swarm king-she's determined to win Jason back.
Realizing that Phoebe's under a spell, Paige confronts Richard. He admits that he cast the spell, but he never meant for it to affect Phoebe. He says he'll make a potion to turn her back to normal, but Paige says she and Piper will take care of it. Concerned about his family history, she doesn't want Richard using magic.
Phoebe approaches Jason while he's on his way to give a speech about a merger with a French company that his conglomerate is acquiring. He tells Phoebe that he doesn't have time to talk to her. Phoebe is furious. Under the influence of Mata Hari, she disrupts the meeting and humiliates Jason.
"Are you trying to ruin me?" Jason asks.
"Oh, that's just the hors d'oeuvre," Phoebe replies. "Wait until you see the entree. It's to die for."
Feeling guilty, Richard makes a potion to strip Phoebe of Mata Hari's bad karma. He knows he can't give it to the sisters to use, so he gives it to Jason instead. Jason hesitates to take it at first, but Richard says, "The only way to fix this is to use this on Phoebe." He drops the potion into the pocket of Jason's shirt. "If you love her, you'll do it."
Piper and Leo manage to get Phoebe home, but she refuses to cooperate. When a group of swarm demons shimmer in and attack, Phoebe helps the demons. They take her to their king.
"Excuse me, are you in charge here?" Phoebe asks.
"I am," the Swarm King says.
"Good, because I'd like to get in bed with you," Phoebe replies. "Not literally, although there may be time for that later. What I'm saying is, if you'll help me, I'll help you." Phoebe, under Mata Hari's influence, wants revenge on Jason for betraying her. And by killing an innocent, she'll bring bad karma to the Charmed Ones, making them easier for the swarm demons to defeat.
While Piper and Paige search for Phoebe in the catacombs, the swarm demons kidnap Jason. He's about to be executed by demonic firing squad when he remembers the potion. He throws it at Phoebe, stripping her of Mata Hari's karma. Phoebe channels the fireballs aimed at Jason back at the demons who threw them. Attracted by the commotion, Piper and Paige enter. With Phoebe's help, they say the spell that vanquishes the Swarm King and all his subjects.
Jason tells Phoebe they need some time apart. She wants to help undo the damage she did to his business deal, but he says he can handle it. "What you and your sisters do," Jason says, "what I saw you do, it put some perspective on my work. I want to protect that."
"You might lose the merger," she protests.
"I've lost worst," he says, then kisses her goodbye.