Chris orbs Phoebe from Hong Kong to help Piper vanquish a slime demon. Since the demon is attracted to magic, Phoebe levitates to lure it out, and Piper blows it up. They don't realize, though, that while the slime was blown to pieces, it wasn't vanquished. They carry bits of the slime home on their clothes, and it begins to reform.
At the manor, Paige and Phoebe go upstairs to their old rooms to pick up a few things, only to discover that Piper has converted Phoebe's bedroom to an exercise room, and Paige's bedroom to Wyatt's nursery. Piper says she packed all their stuff into boxes and put them in the attic.
As the sisters go through the boxes, they come across one marked "Penny." It contains some old clothes that belonged to Grams. Phoebe pulls out a pair of red boots. They're Paige's size, and Phoebe convinces her to try them on. The next instant, Paige disappears in a pouf of smoke.
Paige finds herself in 1967 with a young Penny Halliwell. "You brought my go-go boots," Penny says. "Right on."
"Actually, they brought me," Paige explains.
"That's because I cast a return-to-owner spell on all my favorite stuff. I'm always taking my clothes off and forgetting them somewhere these days."
Meanwhile, Piper and Phoebe summon Grams, hoping she can tell them what happened to Paige. "We've got a problem," Grams says. "Those boots took Paige back to the summer of love, which was one of the most crucial times of my life…Your grandfather, Allen, rest his soul, was a sweet man, but he lead me straight down the hippy-dippy trail."
Grams says that during that time, her best friend Robyn killed Allen, and Grams walked in right after it happened. "My peacenik days ended fast, along with that bitch Robyn." Her rage at Robyn made Grams realize the only way to deal with evil was to destroy it. Grams warns Piper and Phoebe that they have to go back in time and stop Paige from changing history.
Back in 1967, Penny and Robyn are organizing a magical "be-in," a gathering of witches from several local covens. They're planning to tap into the spiritual nexus beneath the manor and "ride the magical wave."
A warlock blinks in and freezes everyone in the room except Robyn. "Everything on schedule for tonight?" the warlock asks.
"They're lining up like lambs to the slaughter," Robyn says.
Piper and Phoebe put on some of Grams's old clothes and are transported to 1967. They find Paige in a park with Penny and a large gathering of witches. The police arrive to control the crowd.
"This is a park, man," someone yells. "You can't control God's green earth."
"It's the taxpayers' green dollars that pay for this park," an officer replies, "and you're loitering. So move it."
The crowd begins chanting, "Hell no, we won't go! Hell no, we won't go!" Penny casts a spell that turns the officers' billy clubs into bouquets of daisies.
"Let's jam," Robyn says. "If the cops catch us, we'll miss tonight."
Piper tries to freeze the officers so she and Phoebe can escape, but her magic doesn't work. (She hasn't been born yet, so her powers don't exist). She and Phoebe are carted off to jail. But Piper casts a spell on the guard, and she and Phoebe are able to break out. They rush to find Paige so they can warn her not to prevent Robyn from killing Allen.
Meanwhile, Leo, Chris, and Grams are trying to contain the slime demon. It's inside the walls and the floors, and it's covering the doors and windows so no one can escape. Leo and Chris don't dare orb, because the demon is attracted to magic. They discover that the demon has spread to the basement, and it's feeding on the nexus.
Allen discovers Robyn reciting a spell to bring evil to the manor. She attacks him, but Paige intervenes, saving Allen and vanquishing Robyn. Piper and Phoebe arrive just in time to discover that Paige has changed their future. Grams is still a flower-child instead of a demon-fighting witch.
As Leo and Chris discuss what to do about the slime demon in the basement, Grams enters dressed in a tie-dyed blouse singing "We Shall Not Be Moved." She says to Leo and Chris, "I think that we should take the demon into our arms and make it feel safe."
The Charmed Ones try to convince Penny and Allen that they need to destroy the warlock that Robyn was working with. "It's not who we are," Penny protests. "We don't believe in murder."
"No," Phoebe says, "neither do we, but we do believe in vanquishing evil."
"I'm sorry," Penny replies, "I don't see violence as a solution to anything."
Unable to convince Penny to help them, the sisters look through the Book of Shadows for potions to vanquish the warlock. They know that if Allen was meant to die, Death will come after him again. They want to make sure Penny isn't also killed in the process.
Chris and Leo work on a device to electrocute the slime demon, hoping to hold it at bay long enough for the sisters to come back and vanquish it. "No offense, you know," Grams says, "but my dear Allen would have taken the path of peaceful resistance. Have you tried talking to the slime?"
"No, I haven't," Leo says, "and you shouldn't either." But before he can stop her, Grams heads for the basement. The slime demon grabs her and pulls her down into its midst.
Allen and Penny send the other witches home, out of harm's way. The warlock soon attacks. Penny tries to cast a love spell on him, but he resists it. He throws a fireball and kills Allen. The sisters use their potions to fight off the warlock while Penny weeps helplessly over Allen's body. The warlock blinks out, but the sisters know he'll be back.
Penny decides to makes a potion to bind her powers. "What good is my magic if it can't save my family?"
"Magic can save your family," Phoebe says. The sisters reveal their identities, explaining that Penny kept her granddaughters safe by fighting off the demons who came to destroy them. This time, when the warlock blinks in, Penny uses telekinesis against him, and says a spell to vanquish him:
"Snuff this warlock, his days are done, but make him good for the ecosystem." He dissolves in a shower of petals.
The sisters return to their own time and vanquish the slime demon with the Power of Three. Phoebe asks where Grams is. With a grim expression, Leo says, "She didn't make it."
"Nonsense." The basement door opens, and the familiar Grams appears, coughing and fanning away the smoke from the vanquish. "You can't damage an old war horse like me. I'm already dead."