Agnes-Marie "Mooney" Pottie is a 15-year-old girl, often seen as the black sheep of her family, who is growing increasingly weary of life in the isolated coastal community of New Waterford. She's recognized as an exceptional student by her depressive, semi-alcoholic English teacher, Cecil Sweeney, who, unfortunately, harbors an inappropriate crush on her that is not returned. Believing in her potential, Cecil encourages Mooney to attend an arts school in New York City. Although she secures a scholarship, her religious parents, Francis and Cookie, flatly refuse to allow her to go.
The dynamics change when a family from New York City moves in next door, introducing Mooney to Lou, the eldest daughter. Lou, whose father is a jailed boxer, may be small in stature but possesses an incredible ability to knock men out when they are lying, which the devoutly Catholic locals perceive as a type of religious miracle. Lou capitalizes on this talent by providing a unique service—knocking out the unfaithful men of the town for a fee from their wronged girlfriends.
As Mooney devises a plan to escape her stifling hometown, she begins to openly kiss various boys in an attempt to create the illusion of promiscuity, all while maintaining her own chastity. This reckless strategy leads to many boys claiming they've slept with her. To further her scheme, Mooney tells her parents she's pregnant, fully aware that this declaration will prompt them to send her away, clearing her path to Manhattan. However, the boys catch on to her tactic, issuing threats to expose her deception. In a moment of desperation, Mooney commands Lou to knock them out as proof of their lies; however, the boys remain standing after being hit, confirming the truth.
Joey, one of the boys, confronts the girls with an ultimatum—Lou must battle New Waterford’s reigning boxing champion in a match, or Mooney's secret will become public knowledge. Against all odds, Lou triumphs in the fight, but Mooney's troubles grow as Joey decides to betray her by revealing her supposed promiscuity to her father. To his shock, Francis mistakenly assumes that Mooney’s pregnancy is a cover for the truth about her relationship with Cecil, leading him to confront the teacher at his precariously situated trailer.
In a dramatic turn, Cecil, realizing the implications, claims paternity of Mooney's child and kisses her passionately. Just then, Mooney’s mother arrives, catching the intimate moment, and in a fit of rage, crashes her car into Cecil’s trailer, sending it plummeting off the cliff to the beach below.
Later, Mooney finds herself on a train to Antigonish, Nova Scotia, where she intends to spend the remainder of her fabricated pregnancy. As she bids farewell to her family, Mooney receives a heart-wrenching letter from her mother. Faced with the tears of her family, she suddenly declares her desire to stay, but her mother insists she return to the train. Onboard, Mooney opens the letter, which uncovers her mother’s awareness of Mooney’s true intentions and ends with a heartfelt wish for her daughter's success.
