Foreign Affairs follows two professors of English literature at an Ivy League college on study leave in London. Vinnie Miner is a tenured professor, who specializes in children's literature. A Fifty-four year old divorcee, she is convinced that her physical plainness will prevent her from ever experiencing true love. Vinnie's strong libido and Victorian mind-set war within her and she has worked out patterns of behavior that allow her to get through life with minimal problems. An imaginary dog Fido, who appears whenever self-pity strikes Vinnie, sizes himself to fit her mood. Childless and not particularly sympathetic to children as individuals, Vinnie will spend six months in a country she truly loves, trying to prove her thesis that British rhymes are more ancient and more literary than American ones, while both share a common theme of violence.

The second academic is Vinnie's younger colleague, Fred Turner, who is not yet tenured. Good looks have been a curse to him from youth and his impetuous marriage to a radical photographer, Roo, has broken down just before his departure. Alone and desperately broke, Fred cannot focus on his work. Through Vinnie, he unintentionally meets a beautiful British actress, Lady Rosemary Radley and falls madly in love. Rosemary introduces Turner to her social circle, changing his dim views on modern English culture, while distracting him from his research. Mutual friends ask him to intervene with Rosemary to hire a maid to clean her disheveled house. Surprisingly, she complies and it is transformed into a wonderful site for a party. When Fred refuses to break his commitment to teach summer school, Rosemary breaks off their relationship and throws him out of the house. Demolished emotionally, Fred asks Vinnie to intervene and explain to Rosemary the pragmatic demands of academic life.

Vinnie, meanwhile, has run into an unemployed, uneducated, garish Oklahoman, Chuck Mumpson. Seatmates on the transatlantic flight, Chuck helps Vinnie find a taxi to get to her rented flat. Because he appears depressed, Vinnie agrees to have coffee with him. Chuck is in England ostensibly as a tourist but is actually on a quest to find a relative, "Old Mumpson," the "Hermit of South Leigh," about whom his grandfather used to brag. Vinnie encourages the project and is attracted by its folkloric aspects, while retaining her disdain for the loud, overweight searcher. Repeated contacts turn them into friends and eventually into lovers, as Chuck reveals the tragedies of his upbringing and dying marriage. Having just begun to enjoy life, Chuck is struck down by a heart attack and Vinnie learns the details from his daughter Barbie, who resembles him closely.

Vinnie receives a phone call from Fred's estranged wife in America, who wants to reconcile with her husband during the brief time both will be in New York. Roo asks Vinnie to have Fred call her at her father's apartment, giving the number for Vinnie's literary nemesis, L. D. Zimmern. Tempted to get vengeance by keeping this secret, Vinnie is empowered by Chuck's loving spirit to trek across London at night and deliver the message.

On the eve of his departure home, lovelorn Fred lets himself into Rosemary's mansion to retrieve clothing and a book. The house has been sacked. Hearing noise in the basement, Fred investigates and confronts Rosemary's Cockney charlady, Mrs. Harris, in a drunken rage. Following him upstairs, she repeats phrases Rosemary used with him in private and when he attempts to leave, she tries to block his panicked exit. Pushing her aside, Fred is disgusted by the sight of her naked breasts, but at home realizes they are Rosemary's breasts, down to the tiniest detail. Mrs. Harris and Rosemary are one. The actress has become her role. Edwin, a mutual friend, confides that Rosemary has a history of psychotic episodes and takes control of her care. Fred is told to forget her and leave her alone. Fred races to the airport just in time to catch his flight.

Edwin and Vinnie meet for lunch and she admits out loud that she loved Chuck. Edwin reports that Rosemary is on the way to recovery and Vinnie reveals a note from Fred saying his marriage is being put back together. As she returns to her flat, Vinnie reminds herself that despite the loneliness she will face when she returns to America, she is a successful writer and she invites a miniaturized and hesitant Fido into the flat.