In the town of Westfield, New Jersey, a high school senior, Katherine, finds herself strongly attracted to Michael, a boy she meets at a New Year's party.

Michael teaches Katherine how to hold his penis (nicknamed "Ralph") and how to rub it correctly, stroking rhythmically, until he ejaculates.

(The book explains that Michael shows Katherine "how to hold him, [moving her] hand up and down according to his rhythm.

A few weeks later, after starting to use the birth control "pill", following her grandmother's frank advice, Katherine reaches orgasm when she and Michael are having sex.

Katherine finds herself aware of the limitations of the relationship and is ultimately attracted to a tennis instructor, Theo, who is older than Michael.

One of the novel's central plotlines is her decision to lose her virginity to him, as well as the sexual relationship they share together.

He is a talented actor who wins a scholarship to a drama academy, and a repressed homosexual who does not respond to Erica's romantic/sexual signals.

Minor Sybil Davison Katherine's friend (and Erica's cousin) who later gets pregnant when having loveless intercourse because she wants to have experience in giving birth.

Ivan Gross Katherine's maternal grandfather, who has had a stroke; he must walk with a cane and is sometimes at a loss for words.

Theo An older boy who Katherine inadvertently falls for while working at a camp, which spells the end of her relationship with Michael.

[4] Forever... has frequently been banned in schools and libraries on due to its detailed depictions of sexual intercourse, and because the protagonist, Katherine, uses birth control.

[11][12] In August 2024, it was one of 13 books banned statewide by Utah's state board of education, allegedly for its "objective sensitive material.

"[13][14] In 2024 the book was banned in Texas by the Katy Independent School District on the basis that the novel is "adopting, supporting, or promoting gender fluidity"[15] despite also pronouncing a bullying policy that protects infringements on the rights of the student.

[16] The book was adapted for American television in 1978, featuring Stephanie Zimbalist as Katherine and Dean Butler as Michael.

In June 2004, the Sacred Fools Theater Company performed a comic adaption of Forever for their Get Lit!

[17] In November 2022, Netflix announced that it had commissioned a television series adaptation, with Mara Brock Akil serving as showrunner and executive producer under her overall deal.