Frances was from a rich family in Somerset, England, who sent her to an expensive private school and she was the school beauty and many boys liked her. She was also popular with many girls such as Liz and they often hung out and messed about smoking in the corridors and other stuff.
Frances was dating Geoffrey and Liz got jealous of Frances' beauty and popularity and thought that if she was out of the way then maybe Liz could be the beautiful girl for a change. Liz began to feel she was sidelined with Frances and that boys were ignoring her. Liz had a lust for Michael, another boy, and she told Martyn of her love for Michael, so Martyn decided to arrange a quadruple date for them all and leave them in an old Army underground complex for a weekend then come and get them the next Monday.
The whole film is told from Liz's time in the police station because the police went and broke down Martyn's door when he didn't cooperate and then he turned up drowned, which they believed was a suicide. However, the police woman in charge of Liz begins to realize the girl is mentally ill when she makes up a fake story of them all surviving.
Frances is first shown with Liz messing about in their glamorous dormitory (all pink) in the academy and smoking and laughing about boys. Later, when Liz tells Martyn about her love for Mike, he arranges a weekend date for them all, and Liz tells Frances, who goes publically into the showers to tell Geoffrey. All the jocks laugh at her for being so daring and she gives them the finger. Frances then gets the place deserted except for her and Geoffrey (who is nearly naked) and she tempts him into coming with her in the Hole and she says if he's too cowardly she'll ask someone else. He eventually agrees to come and asks if he can do anything else which Frances says "Yeah, put your knob away."
Frances then goes with Geoffrey after school to the Hole with Liz and Mike, and Martyn shuts them in after saying he'll be back on Monday. For a few hours, the trip is fun. Frances dances around showing her nearly naked body and attracting Geoffrey and also attracts Mike, to Liz's irritation, and Geoffrey then professes his lust for Frances by saying he wanted to lick her naked stomach for four years, and then he does so by getting her lying down. Mike comes over to watch and when Liz sees Mike holding France's arms back Liz thinks he's cheating on her so she storms up to Mike and slams him on the head. Frances applauds Liz because she jokes she saved her from being raped.
Just after, Mike is getting all horny so Frances gives him what he wants and yanks down her tank top showing him her naked breasts. Mike applauds and Geoffrey asks what he missed. This has become an infamous scene in the film.
Frances is shown that night making out with Geoffrey, giggling lustfully in the background, annoying Mike who yells he's trying to sleep alright. Liz then shows her naked body to Mike but he's uninterested and rolls over.
When Sunday comes and goes, they wait impatiently for Martyn to come and rescue them. They hear footsteps over the Hole but nobody opens it up. Then Liz and Mike try to open it but can't. Frances then begins to grow slowly ill after food runs out, and they begin to realize Martyn cold-bloodedly abandoned them and he is not coming for them.
Frances already has belumia and she is already very thin because of her eating disorder and gets weaker as the food has almost gone. Frances is soon coughing over the toilet and vomiting compulsively. Liz begins to get happier as Frances gets weaker and at one point Liz even says sadistically that she'll make out with Mike overnight when Frances is vomiting repeatedly into the toilet.
Later in the night, Liz wakes up and calls for Frances but she doesn't answer. When she goes into the toilet she finds Frances dead, choked on her vomit. Liz pretends to scream in shock, but it is later revealed Liz deliberately let Frances die just because she had enough of her. Geoffrey blames Liz for Frances' death, rightly so.
They stage a mock funeral for Frances and later she is taken by police to the morgue and presumably given a proper burial.