During World War II, Pat Jamieson, a lovelorn scientist from Chicago, travels to Washington, D.C.  to work on a secret project to develop a high altitude oxygen helmet for the War Department.  Pat is a stranger to Washington, but he soon finds a temporary home for himself and his cairn terrier, Dizzy, through a chance acquaintance with a drunken man named Quentin Ladd.  Quentin takes Pat to his cousin Jamie Rowan's elegant house on Connecticut Avenue and casually invites him to stay the night.

The next morning, Quentin finds Pat tinkering with some scientific instruments in the basement of the house and realizes that his guest intends to stay.  During the course of the day, Pat learns from Quentin that Jamie is a lonely and unhappy woman, and that she is trying to sell the house.  Pat wants to use the basement as his laboratory but because he insults Edwina Collins, Quentin's overbearing and snobbish fiancée, he is asked to leave the house at once.  On his way out, Pat meets Jamie, who mistakes him for an applicant seeking a job as a caretaker at the house.  Thinking fast, Pat feigns interest in the job and uses his unique charm to win Jamie's approval.

Pat's ruse is exposed, though, when Edwina discovers that he is a respected scientist, as well as the son of a well-known scientist.  When Jamie realizes that her father and Pat's father were old friends, she drops her cold indifference toward him and invites him to stay in her home.  Pat insists on a strictly professional arrangement, as caretaker, but Jamie soon takes a personal interest in him.  As Jamie and Pat become better acquainted, they realize that they share a distaste for the romantic notion of love, having both been hurt by past experiences.

One day, Kitty Trimble, Jamie's hard-boiled business manager, shows the house to Paul Carrell, a prospective buyer, who, by coincidence, is an old friend of Pat.  Paul flirts with Jamie, and Jamie later admits to Pat that Paul's visit has had a strange effect upon her.  After deciding that she must stop living in the past, Jamie, emboldened by her new outlook on life, proposes marriage to Pat.  She insists, however, that theirs should be a marriage uncomplicated by love and based entirely on trust, companionship and humor.  Pat accepts Jamie's proposal, and they develop a Platonic relationship, deliberately devoid of intimacy.  They also develop a professional relationship when Jamie begins to assist Pat in his experiments.

Some time later, Paul privately questions Jamie about her marriage, accusing her of entering a loveless union, then plants a kiss on her.  Jamie angrily protests the kiss and, in so doing, blurts out that she loves Pat.  The sudden realization that she is in love prompts Jamie to follow Pat to Chicago, where he is about to give his first official presentation of his oxygen mask.  The demonstration is a success, but a series of jealous misunderstandings results in Jamie's early return to Washington.  While Pat remains in Chicago for a few days, Jamie meets Pat's French ex-girl friend and tries to adopt her seductive style to impress Pat when he returns.  Pat does not appreciate the change in Jamie's behavior, but when he confesses that he is in love with her, she reverts to her old self and gives Pat a big hug.