Piper and Chris are waiting in a sawmill, where a mysterious Slime Demon is skulking about. Chris isn't happy that Paige is running late, and that Piper let Paige and Phoebe move out. Paige now lives at Richard's place, while Phoebe has moved to Hong Kong to be with Jason. As a result, Piper has had to vanquish four demons in a row by herself. Darryl holds off a crowd outside. Chris orbs to Hong Kong to fetch Phoebe. Phoebe levitates to lure the demon out, since it senses magic and feeds off of it. The demon breaks free from the floor and Piper blows it up.
Back at the manor, Piper is soaking Phoebe's clothes in the sink when Paige orbs in. She's greeted by an excited Phoebe who asks how Richard is. Paige claims he's fine, and that she's agreed not to use her magic in front of him. Phoebe asks about Chris, and Piper says he's still shaken up from what he saw back in his time. The death of his fiancee, Bianca, has made him more determined than ever to find out what turned Wyatt evil in the future. Phoebe wants to change her clothes at the manor. When Piper puts the dirty clothes in the wash, a small green blob crawls out of the washing machine and into the vent.
Phoebe and Paige go to Phoebe's room only to find that Piper has turned the room into an exercise room, while Paige's room is now Wyatt's room. Leo is holding Wyatt, and appears to be more willing to help Chris protect Wyatt. Phoebe and Paige are going through their belongings in the attic as the converse about adjusting to the change. Piper finds a pair of red go-go boots. They're in a box marked "Penny." Piper and Phoebe find it hard to believe Grams ever wore anything that stylish, even in her younger years. Piper notices that the boots are Paige's size, and talks a wary Paige into trying them on. Paige disappears in a puff of smoke, and reappears in a room with a young hippie woman. The woman says the boots are hers. She explains she cast a return-to-owner spell on many of her clothes because she was always mislaying them. The woman says her name is Penny. Paige realizes that the woman is her grandmother. She also notices the date--January 1967. Another young hippie, Robin, pokes her head in the door and says that a bunch of Whitelighters are putting on a light show. Penny invites Paige to go with them.
In the present, Piper, Phoebe explain to Leo and Chris what happened. Leo states he and Grams met once back in the 60s and that she used to cast spells on her clothes when she was young. Chris suggests summoning Grams for help.
In 1967, Paige, Penny and Robin watch the Whitelighter light show with several other witches. Paige thinks she recognizes one of the Whitelighters as Leo. Paige asks who everyone is, Penny and Robin explain that they are all witches and whitelighters. Penny says they're getting ready for a "magical be-in" that night at the manor. Paige reveals she can orb--but when she tries to orb an 8-track player, but nothing happens. Leo offers to give Paige some "private coaching," but Paige balks. Paige expresses her amazement of all the witches present in one place. Penny says that they plan to tap into the power of the Nexus at the be-in. Suddenly, the entire room freezes--except for Robin. A warlock, Nigel, blinks in. He and Robin talk about plans to wipe out everyone at the "be-in" that night. Robin's voice tone and demeanor change noticeably when she talks with Nigel; she hates having to pretend to be a hippie. Nigel blinks out, and the room unfreezes. Penny wants Paige to join them for the "be-in," but Paige only wants Penny to write a spell so she can get back home. Just then, a horn honks and Penny leaves to greet her husband, Allen, as he pulls up in a rainbow van. Allen dropped Patty off at his sister Janice's place. Allen meets Paige and thinks she's a dead ringer for his sister. Paige expressed that she would like to go see the Book of Shadows. Allen and Penny talk Paige into coming with them to a drum circle in the park.
In the present, Piper summons Grams. The green blob has stuck to Chris's shoe and grows to cover him. Leo suggests that Chris orb out, but Chris isn't sure it's a good idea. The blob grows stronger as Piper summoned Grams. Grams, Piper and Phoebe finally talk Chris into briefly, orbing out while Piper blasts the blob- which retreats into the floorboards.
Grams is angry that the sisters are not living together, and is particularly displeased that Piper let Phoebe and Paige move out. Piper and Phoebe are defiant, saying they did it for love, but Grams is somewhat ambivalent toward men. Chris hears the blob within the wall and notes that in his future, it had taken the Power of Three to vanquish it. Grams suggests that Leo take the lead in finding a way to vanquish the Slime Demon, thinking Chris is too green.
Grams explains where the red go-go boots took Paige. She tells the sisters what happened to her in 1967. Allen had led her into the hippie lifestyle, but she was shaken out of it when Robin, her best friend, murdered him. Grams didn't know it at the time, but Robin was an evil witch. She explains that she had cast a return-to-owner spell on several of her clothes during that time. She has Piper and Phoebe put on some of her hippie clothes so they can travel back in time. Meanwhile, Leo, Chris and Penny find a way to vanquish the slime demon.
Paige is gathered with Penny, Allen, Robin and a bunch of other hippies at the park. Piper and Phoebe appear in the park. Just when they reunite with Paige, the cops show up and try to break up the circle. The hippies won't budge, and the cops break out their batons. Penny chants a spell and turns the batons into bouquets of flowers. The cops cuff Piper and Phoebe and hustle them to jail. Penny takes Paige with her and Allen to avoid being sent to jail.
In jail, Piper realizes that this is the night Allen dies. Phoebe hopes that Paige figures out a way to get them out. A black man with an Afro, Luther Morris, is making a ruckus in a nearby cell. Piper recognizes him as Darryl's father.
Back at the manor, Paige is worried for her sisters, but Allen tells her they'll be out the next morning. Penny comes downstairs with some pyrite crystals. Robin asks what they're for, and Paige explains that they're used to ward off evil. Robin wonders why they need crystals if the "be-in" is going to be peaceful, but Penny explains she doesn't want to take any chances in case any evil covens try to crash the party. Paige expresses her urgency about her sisters, Allen thinks a lawyer friend of his can help get Piper and Phoebe out of jail.
In the present, Leo, Chris and Grams try to figure out how to rein in the slime demon. Chris explains no one in the future could control it. They argue about ways to defeat it when Wyatt orbs his bottle which causes the slime attacks again. They try to take Wyatt to safety, but the slime has them trapped inside the manor.
In the past, Piper and Phoebe attempt to escape jail by casting a spell to make Phoebe irresistible to a guard. Phoebe seduces him into the cell and the sisters escape with Luther.
Back at the manor, Leo again offers to give Paige a "private session," but Paige balks. She asks where to find Allen, and Leo continues to hit on her. She asks for Allen again and Leo points upstairs. In the attic, Allen finds Robin chanting a spell to unleash evil in the manor. Robin readies a fireball, but Paige rushes in just as Robin is about to throw. Robin throws the fireball at Paige, but Paige dives out of the way. Allen tries to talk Robin into giving up evil, but Robin won't hear of it. Paige knocks over a can of marbles just as Robin is about to throw another fireball. Robin slips and falls, sending her own fireball straight up into the air. Piper and Phoebe rush in just as Robin's own fireball falls full force on her, vanquishing her. Paige, Piper, and Phoebe realize Paige may have changed the future or erased it.
In the present, Leo and Chris try to make an escape plan. They ask Grams for help and notice she has suddenly become an aging hippie. Leo and Chris realize that the sisters somehow altered the timeline. The slime is still growing. As Leo takes Penny and Wyatt to the attic, Chris get things they might need.
In the past, Penny walks in the attic to find Allen and the sisters. Allen tells the sisters and Penny about Robin and Nigel's planned massacre. Piper and Phoebe think that between all of the witches there, they have more than enough magic to vanquish Nigel. However, Penny won't hear of it; she doesn't believe in murder. Allen suggests calling the "be-in" off to prevent anyone from being hurt. Once they leave, Paige suggests that this might be Grams' new destiny. Phoebe points out that they weren't supposed to change anything. She adds that if this is the night Allen is supposed to die, nothing will stop it. Phoebe and Paige leaf through the Book of Shadows to find a potion while Piper stays downstairs in case Nigel shows up.
In the present, Leo and Chris try to get the slime to attack a pile of electrical appliances. Grams thinks they should try talking to it instead. Leo adds a toaster oven to the pile, and ends up blowing a fuse. Chris goes to the basement to get to the fuse box, and finds the slime is there. The slime has grown to immense size, and is feeding on the Nexus. Leo takes a look and see it has almost flooded the basement. The Slime the tries to attack Leo but he quickly slams the door.
In the past, as everyone is going home, Piper tries to reason with Penny and Allen. However, they still aren't willing to fight Nigel. Piper tells Penny that her grandmother taught her that they have to vanquish evil, not reason with it. Penny and Allen are horrified; they don't think it's possible to be good and kill at the same time. Piper asks if they would fight if someone evil attacked someone they loved. The room freezes and Nigel blinks in. Not seeing any other witches around, he unfreezes the room. Allen offers to surrender, but Nigel knocks him across the room. Penny tries to cast a love spell on Nigel, but it only makes Nigel angrier. He tries to kill Penny with a fireball, but Allen knocks Piper and Penny to the floor while taking the fireball for them. Penny begins to weep for her dead husband. Nigel is about to kill Penny as well, but Paige knocks him across the room with a potion. Phoebe throws another, but Nigel blinks away. The sisters notice Penny crying over Allen's body and realize something isn't right. Piper and Phoebe remember that Grams flew into a rage at Allen's murder instead of crying.
In the present, Chris has two wires at the ready to zap the slime. Grams tries to pacify it--and is sucked in.
In the past, Penny is making a potion to bind her powers, blaming herself for Allen's death. The sister try to persuade her not to. Phoebe accidentally lets out that they are sisters and Penny asks who they really are. The sisters admit that they're Penny's granddaughters, and that her enchanted clothes were what brought them there. Penny doesn't believe them at first. Nigel blinks back in and blasts the remaining potion with a fireball. Phoebe attempts to kick Nigel, but he blinks out of the way.
In the present, Leo and Chris hold the door back to prevent the slime from coming out, Chris tries to ward off the slime by zapping it. He and Leo realize all it really wanted was the Nexus. Chris doesn't know if he can hold out much longer, but Leo thinks they have to for Penny's sake.
In the past, Nigel sends Piper and Paige flying into a coffee table. He readies a fireball and is about to destroy them when an enraged Penny comes up behind him. She violently flings the warlock around the house, then smashes a table and the grandfather clock onto him. She finishes him off by blasting him into a shower of daisies. Penny dedicates her vanquish to Allen. Penny reverses the spells on her belongings and the sisters return to the present.
The sisters vanquish the slime-demon with a spell. Phoebe asks where Grams is. Leo and Chris fear Grams didn't make it. Just then, Grams comes out of the basement, back to her old self. Paige taunts Leo about their past encounter, but an embarrassed Leo asks Paige to keep that between them. Grams congratulates Leo and Chris for not giving up.
Grams gives Phoebe and Paige a severe tongue-lashing, but gives them her blessing to move out. However, she makes Phoebe promise to keep in touch more and makes Paige promise to use her magic. Piper says they'll always have a room at the manor if they ever choose to come back. Paige orbs Phoebe home. Leo walks in and asks if everything is alright. Piper says she wishes they still lived together. She leaves to go to sleep. Leo says goodnight to Wyatt. As the episode ends, we see Wyatt's blanket has a giant peace sign on it.