Jan Morrow, Doris Day, is an interior decorator in New York City. She lives alone and, when questioned by her drunken housekeeper Alma, Thelma Ritter, she calmly asserts that she’s content with her solitary life. The one recurring irritant in her routine is the party line she shares with Brad Allen, Rock Hudson, a charismatic Broadway composer and notorious playboy who lives in a nearby apartment building. The telephone company itself is overwhelmed by a flood of new requests, so Jan can’t obtain a private line to run her home-based business.
From the start, Jan and Brad have never met in person; their disagreement and banter unfold over the shared line. Brad uses the phone to flirt with one woman after another, serenading each “in love” with songs that he insists are original, though the tune remains the same with only the names or languages altered. Jan needs the line for work and tries to win him over to a more considerate habit. Brad toys with her by insinuating that she’s secretly jealous of his popularity and even teases her about “the bedroom department.” His charm works on others, including a telephone inspector, whom he wins over with his easy manner.
Eventually, the two strike a compromise: they agree to share the line on alternating half-hour blocks. Jan also takes on a new client, millionaire Jonathan Forbes, Tony Randall, who repeatedly asks her to marry him. Unbeknownst to Jan, Forbes is Brad’s old college pal and the current Broadway financier, and he lets Brad know about the latest romantic target. Brad becomes intrigued by the idea of Jan as a challenge and potential partner.
One night at a nightclub, Brad spots Jan dancing and learns her identity. He crafts a pretending persona, Rex Stetson, a wealthy Texan rancher, adopting a thick Texas accent to woo her. He tests the waters by letting Jan believe Rex might favor her, including a staged moment where Rex would stop by his hotel room to charm her, only to show up with just a view to offer. Jan, feeling flattered, brags about the encounter, and Brad slyly hints that Rex could be gay, prompting Jan to question Rex’s real preferences. The two eventually share a kiss, drawn together by the ruse and the chemistry between them.
For Forbes, a private eye is hired to uncover who Jan is dating. When Forbes discovers the truth about Brad’s masquerade, he orders Brad to take Jan home by taxi and then to Connecticut to meet him at a cabin to finish his new songs. Brad, however, pulls a swift double cross by inviting Jan to join him at the cabin. There, after a kiss or two, Jan stumbles upon Rex’s sheet music, realizes the melody is Brad’s, and confronts him. She asks to be taken home by Forbes, who has just arrived to end Forbes’s manipulative schemes.
Back in New York, Forbes is less in love with Jan because she cries over Brad so easily, but he’s secretly pleased to learn that the charming playboy has fallen for someone. Brad, meanwhile, seeks guidance from Alma. She, having listened in on the party line, suggests an unusual plan: Brad should hire Jan to decorate his apartment so they will be forced to collaborate, all while Brad uses their mutual work to “drink under the table” with her. Jan agrees, mainly to protect her commission, and Brad lets her handle the design details.
Brad’s apartment becomes the battlefield for their creative clash, with Jan choosing the boldest, gaudiest décor she can imagine. When Brad sees the transformation, he storms into her space, retrieves her in her pajamas, and carries her through the street back to his place to demand explanations. He laments the changes he’d made in his bachelor lifestyle in the belief they would lead to marriage, and as he leaves in anger, Jan sees a chance to prove herself. She triggers one of his playfully mechanical switches, and the player piano in his room pounds out a wildly honky-tonk version of his signature love song. Their eyes meet, and they finally embrace.
In the aftermath, a new chapter begins. A few weeks or months later, Brad heads to tell Forbes he’s going to be a father, only to be pulled away by Dr. Maxwell, an obstetrician, and Nurse Resnick for an examination, as he discovers he is going to have a baby. The scene closes on the promise of a future that blends work, romance, and a shared life that neither could have predicted at the start of their comical, heartfelt feud.
