Summer of 1969. As their anniversary approaches, 17-year-old Mary Claire Denning and her boyfriend, Corrigan Bloom, decide to go on a romantic getaway to see their favorite musicians, the rock duo The Blamies, on a big concert named The Trip, which is about to take place all across the country, in California. Knowing that her over-protective mother won't let her go, Mary Claire decides to escape, leaving her a letter explaining where she is going. She also decides to take her cat, Ina, along with them, and the young couple goes on their adventure... completely unaware that The Blamies have gotten into a big fight and that both musicians, vocalist Brie Rickman and guitar player Liam Buckson, have disappeared.
While touring during their travel, Corrigan accidentally loses Ina and, as the couple tries to find her, they lose their train. Ina is found by Hayley Blue, a laid-back hippie who, after hearing about them losing their train, invites them to spend the night in the hippie commune she belongs to. Fascinated by the idea, Mary Claire accepts the proposal, while Corrigan remains doubtful. As a way to thank them for allowing them to stay, Mary Claire offers to cook some pastries and cupcakes for them and, as she searches for the flour in one of the commune's buses, she is accidentally hit by a guitar case that leaves her unconscious.
Informed about The Blamies' break-up, hippie commune leader Seth Caplan is asked by their manager to find them and help them to solve their differences. Seth forms a small group that includes Hayley for said task and leaves the commune... unaware of the unconscious Mary Claire on their bus.
Mary Claire wakes up a few hours later and chaos breaks loose as everyone tries to understand why is she here. Mary Claire asks them to take her back to the commune, but Seth explains that he can't do this given the importance of his mission. As he tells her the details of the mission, a shocked Mary Claire decides to tag along and help Seth and the hippies find The Blamies and bring them together. She calls Corrigan and, since the commune is heading to The Trip as well, they decide to meet at the concert.
Mary Claire travels together with Seth's group to a protest held by social leader and Brie's on-and-off boyfriend Malcolm Holbrook, where they find Brie. Seth lies to Brie, telling her that Liam sent them to solve things out, and convinces her to go with them - which, naturally, fascinates a shocked Mary Claire. Mary Claire develops a strong connection with both Brie and the hippie group, sharing their philosophy and concerns, and becomes known as the "Cupcake Girl", while Corrigan, traveling with the rest of the commune, is incredibly uncomfortable and can't wait to reunite with Mary Claire.
Seth's group finds Liam Buckson in the house of his sister and manager, Sarah. Seth asks the hippies to keep Brie busy while he tries to persuade Liam on his own... but things quickly fall apart as an angry Liam confronts Seth and Sarah for plotting against him. As things come to light, Brie gets upset as well and chaos breaks loose. Liam leaves Sarah's house, and Seth asks Liam's hippie friends, Hendricks and Panama Lewis, to talk him back to his senses, while he, Hayley, and Sarah try to calm Brie. Realizing that Brie is upset for having been lied to, Mary Claire asks Seth to let her talk to Brie by herself. Mary Claire talks with Brie and convinces her to do what she thinks is right for herself and her fans. As Liam is calmed down by Hendricks and Panama Lewis, he comes back to Sarah's house and talks to Brie. They agree to play at the concert, but their personal relationship remains tense.
Corrigan manages to call Mary Claire and informs her that he has convinced the rest of the hippie commune to go to Sarah's place. An excited Mary Claire tries to meet Liam, but he confuses her with a groupie and kisses her. As Corrigan and the rest of the commune arrive the next day, Mary Claire feels uncomfortable and guilty, and asks Corrigan to leave the hippie commune, explaining what happened with Liam. They argue and conclude that it might be better not to go to the concert. As they inform Seth and Hayley about it, Hayley decides to take them to the train station.
At the train station, Hayley tries to convince them to go to The Trip, telling them they have gone a long way to just go back. Mary Claire and Corrigan discuss the reason for their problem and Hayley helps them to solve it. As the young couple agrees to continue their travel to the concert, they are found by Mary Claire's mother and Corrigan's sister (who have been searching for them all along) and they decide to escape, once again.
They find Holbrook outside the train station and he agrees to give them a ride to the concert... even though he doesn't have a ticket and, instead, makes them trespass through a distant zone. Sure enough, they are quickly arrested. Mary Claire and Corrigan show their tickets and are released, but since neither Hayley nor Holbrook have one, Mary Claire and Corrigan search for Seth and Brie (who are already in the concert).
Mary Claire and Corrigan find Seth and his group hanging out with Brie and Liam alongside some other musicians. As Mary Claire tries to tell them that Hayley and Holbrook are in trouble, Corrigan and Liam get into a fight that ends up with the musicians' tent getting burned. An enraged Mary Claire breaks the fight and sets things straight with both Corrigan and Liam. As things calm down, Mary Claire and Corrigan take Seth's group and The Blamies to release Hayley and Holbrook. Noticing how Seth worries about Hayley, Mary Claire advises him to tell her about his feelings and Seth and Hayley end up together.
Mary Claire and Corrigan spend the night with the hippie commune and make love, finally being able to enjoy the concert and adventure. As The Blamies perform a hell of a concert the next day, they are surprised to find out that Brie has composed a brand new song called "Cupcake Girl and The Trip", in honor of Mary Claire and her efforts. Mary Claire and Corrigan enjoy the concert but are then found by Mary Claire's mother and Corrigan's sister, telling them that they are in a lot of trouble. Seth, Hayley, The Blamies, and the rest of the commune defend them, but Mary Claire admits her doing was wrong and apologizes to her mother, while telling her that even though the world might be a scary place, one can't live full of fear or might end up losing the beauty of it. Mary Claire and her mother reconcile and decide to go home.
Mary Claire and Corrigan say goodbye to the hippie commune and the Blamies and leave the concert, thinking about their amazing adventure. Months later, Mary Claire receives a gift from Brie - the new album of The Blamies, which includes her song, "Cupcake Girl and The Trip."