NOTE: This spoiler was written by Nash who says, "If the spoiler seems light on any other characters from the band, that's because the film is as well. It's mainly about Cherie Currie and Joan Jett. The other girls get very little screen time and even less dialogue."
The film starts off following 15-year-old Cherie Currie (Dakota Fanning) hanging out with her sister Marie Currie (Riley Keough) in the San Fernando Valley and getting her first period with period blood running down her leg. They are both skipping school and getting picked up by her sister's older boyfriend. They get picked up and the boyfriend is an obviously really skeevy guy and starts fingering Marie in the passenger seat as Cherie looks on from the back.
Meanwhile, Joan Jett (Kristen Stewart) is loitering at some edgy clothes store. When called out on shopping in the boys section, she walks up to the counter and dumps a bag of change on the counter and asks for what the guy next to the cashier is wearing, a kind of punk rock, leather jacket ensemble.
It's Cherie's birthday and she sits alone in her kitchen with a partially eaten cake when the phone rings. It's her father who disappoints young Cherie by telling her he won't be able to make it home but hopefully will see tomorrow morning. Joan spends her time hanging out with her friend Tammy (Hannah Marks) out on the streets dreaming rock n' roll dreams. Tammy kisses Joan out of nowhere but Joan doesn't seem to mind.
Cherie performs at a school talent show. She dresses up like David Bowie and lip syncs to one of his songs as her schoolmates throw paper and junk at her booing her. She defiantly gives them all the middle finger to raucous cheers and walks off stage.
Joan goes to a guitar teacher who tries to teach her old-timey music with an acoustic guitar, which Joan rejects, plugs her electric guitar into an amp, and starts rocking over the teacher's protests.
Joan goes to a punk rock club and makes out with some guy outside until she sees famed record producer Kim Fowley (Michael Shannon). She strikes up a conversation with him and tells Kim that she plays guitar but lacks a band. She tells him about her dream of starting an all girl rock band. Kim calls another girl over. Her name is Sandy West (Stella Maeve) and she plays drums. He tells them to hang out and rock out a bit and then call him when they have something. Cherie is also seen hanging out at similar punk rock clubs.
Sandy and Joan are practicing over the phone with Kim and he tells them that they're ready to start putting something together. He comes to Sandy's house where he listens to them play as he reads a magazine. He tells them to stop and shows them what they are missing. He points out a striking blonde bombshell model.
Kim and Joan are seen prowling through a club looking for someone that fits that bill and Kim gravitates towards Cherie, hanging out alone in a corner drinking a Mountain Dew. He asks her whether she can sing or play an instrument. She says she can sing and mentions her winning the talent contest by lip-syncing David Bowie. They tell her to learn a song and meet them at a trailer in the Valley to try out.
Once at the trailer, there are three other girls that round out the band. The song she tells them she wants to sing is a slow song that the band doesn't want to play and they tell her to wait outside. Kim calls Joan over and they come up with the lyrics for the song Cherry Bomb. They invite her back in and tell her to sing the song. As she's trying out, Kim tells her to be wilder while singing. She gets in the band. The Runaways are born. Kim constantly tells the girls that they will be bigger than the Beatles.
Cherie takes her bad attitude home with her as she smokes at home with her Marie, who doesn't think the band will take off. Their mom comes home with a guy and tells them some big news. She is moving to Indonesia with her new boyfriend. The girls end up living with their father, who lives with their aunt and grandmother.
So the band practices and Kim trains them to be rougher and to act more like men. He invites kids into the trailer and has them hurl junk and insults at them as they play. They then play their first gig at a house party where the insult training has paid off as they knock the trash thrown at them back at the audience. Meanwhile, Kim is upstairs setting up their first tour, when the cops bust the party up.
Cherie packs up for the tour and says her good-bye to her family as the rest of the band waits outside. No one knows where her father is until they walk out and find him drunkenly passed out in the car outside. He is a major alcoholic. Cherie leaves with the band and lets her sister deal with the mess. Cherie promises Marie she'll take her on the next tour.
They are all cramped into a small station wagon with a trailer carrying their things and equipment. They stop at a hotel and are surprised at the cramped conditions of the room. Cherie checks in on her family as the rest of the girls play at the pool. Cherie and Sandy get into an argument with some older guys that they are opening for while they are doing a sound check. Later on, before they go on, Cherie is having sex with the roadie in the bathroom backstage. Joan and Sandy are stumbling around backstage drunk trying to get into the bathroom but can't get in so they decide to go in the dressing room of the guys they almost got into a fight with earlier. Joan pisses on their guitar and runs out. They bump into Cherie and they all take some pills and go out and play.
Joan calls Kim to get more money but he's too busy having sex with a girl to care but tells her that he is working on big things for them. They continue touring and getting into the usual sex, drugs, and rock n' roll shenanigans. After a gig at a roller-skating rink, Cherie and Joan end up having sex. The morning after, Joan gets a call from Kim and finds out that the Runaways have a recording contract. A montage of them signing contracts, playing gigs, and various magazine clippings starts playing. A photographer comes to Cherie's house and takes very sexy, provocative pictures of her for a magazine. Cherie is packing up at home as she readies for their tour in Japan. Marie suggests jobs that she can do on the tour but Cherie tells her she can't go, even though she promised. Cherie says the other girls aren't taking any family so won't either.
On the plane to Japan, Cherie and Joan go into the bathroom and finish off their drugs before they land. Looks to be mostly cocaine. The Runaways are mobbed by screaming teenage fans. They are huge in Japan. The girls play some great gigs and delve even deeper into drug use, especially Cherie. At one gig, Cherie goes out on stage in very sexy lingerie. After the gig, the girls confront her about a magazine published in Japan with the sexy pictures of her taken at her house. The girls are angry that she's selling herself as a slutty model and that they won't be taken seriously as a band. The girls pounding on the glass in their dressing room finally break through and rush the girls. Cherie, drugged out of her mind and with another guy in her room, gets a call from home. It's Marie. Their father is sick. Cherie doesn't acknowledge this, ignores the call, and hangs up. As Cherie delves deeper into drug use and arguments with the band, she passes out in the hotel lobby. When she wakes up, Joan is with her.
When Cherie comes home, she apologizes to her sister and visits her ailing father. Cherie is getting sick of the rock star life and sees it destroying her regular life. In the recording studio, the band is waiting on Cherie who won't get up from reading a magazine. Joan goes over to see what's so interesting and sees a quote from Kim trashing Cherie and talking about her huge ego. Kim comes into the studio and yells at her to get back to recording. Cherie says no and refuses. She leaves the studio and leaves the band. The band is done. Kim shuts down the studio and tells them that they're done. Joan leaves and finds Cherie still outside. Cherie says she just wants her regular life and her family back. Joan tells her that the band was her family and rock n' roll is her life.
It's eight months later. In an interview with Kim, he says that the Runaways were a grand experiment that failed. Joan is seen having orgies and strumming her guitar with some lowlife friends and her old friend Tammy. She gets sick of everyone and throws everyone out.
The film ends with Cherie working at a garment store, listening to the radio when she hears Joan having an interview on-air. She decides to call in, talking with each other for the first time since the band broke up. After an awkward bit of exchange, it's shown that they both are in really good places right now with Cherie sober and trying to be an actress and Joan continuing her music with her new band, the Blackhearts.
In a text epilogue, we are told that Jett's first solo record was rejected by 23 record companies but ended up selling 10 million copies. Kim Fowley worked with some of the most legendary rock bands in the world and now has green hair and walks with a cane. Cherie Currie grew up to help teens with addiction and has become a chainsaw artist. Nothing is said about the rest of the band.