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Middle-aged New York publisher Barton Kendrick and his wife Diana have a "modern" marriage in which each allows the other romantic freedom.  Diana secretly wishes the situation were different, but Bart revels in his relationship with the beautiful Johnny Jones.  Johnny, a Viennese refugee who has been staying illegally in New York since her temporary visa expired, is in love with Bart but won't accept his marriage proposal because she is worried that Diana will be hurt.

When immigration official Barney Grogan comes to Johnny's apartment to tell her to report for deportation, he informs her, off the record, that if she marries an American citizen she will not be deported, then gives her a week to find a husband.  While Johnny is thinking things over in a diner, she encounters Bill Smith, an aspiring writer who is completely broke.  She gets the idea to marry him and invites herself back to his apartment.  When she proposes, he is reluctant, but eventually agrees and convinces her to draw up a contract whereby she will pay him $17.80 each week.

Two months later, Bill has fallen in love with Johnny.  Meanwhile, Bart knows that Johnny has been granted residency, but she won't tell him how.  One night, when Bart proposes and says that his divorce can be arranged in six weeks, she confesses that she is married.  She then goes to see Bill a day early, and he shows her his novel based on their marriage, Without Love.  She is touched, but wants him to give her a divorce right away, and tells him that she loves someone else.  Though heartbroken, he agrees to the divorce.

After some revisions, Bill sends his unfinished novel to some publishers, one of whom is Bart.  Diana, who reads all of Bart's submissions, calls Bart to tell him about the wonderful new writer she has discovered, and when he reads the unlikely plot, Bart finds it too familiar.  He then invites Bill to his office, and as he staunchly defends the "older man" in Bill's story, Diana realizes that Bart is the real older man.  She convinces Bart to give Bill a $500 advance, and Bill thinks that he now has a chance with Johnny.  Diana then tells Bart that she will give him a divorce as soon as Bart is convinced that Johnny really loves him and not Bill.  Bill goes to see Johnny to repay her and tells her that before he agrees to a divorce, they must take a trip together.

After leaving New York, they stop at a roadside restaurant, where she secretly calls Bart, who promises to come for her right away.  They then drive to a farm owned by Bill's kindly grandmother.  Later that night, while staying in rooms divided by a three-quarter wall, neither Bill or Johnny can get to sleep.  After Bill tells Johnny how fireflies show their love and recites a romantic poem, she falls in love and turns her flashlight on and off like a firefly.  Just then Bart arrives, and Bill soon realizes that Bart is his romantic rival.  The men argue, but after Bill goes to his room, Johnny sends away Bart, who finally realizes that he still loves Diana.  When Johnny goes upstairs, she flashes the light again and they kiss over the wall that bears one of Grandma's Shakespeare-quoting samplers, "All's well that ends well."