Fifty years ago, men all over the world started becoming pregnant. In Japan, there are roughly 40 reported cases a year. Kentaro Hiyama is a 37-year-old advertising executive who has successfully marketed a new campaign for his client called UNIVE, which strives to have the public be more accepting of their looks. Kentaro frequents women, but always comes back to freelance writer Aki Seto who is after a steady job and whom Kentaro views as "mature". Following a night with her, Kentaro begins to feel sick. He vomits at work and goes to get a checkup upon which, to his astonishment, he discovers that he is 9 weeks pregnant. In denial, he hastily buys a pregnancy test which confirms it, before getting a call from his boss, Osugi, telling him that his campaign idea has been accepted, and that he does not have anyone to replace him.

After speaking with her friend, Eri, who has had a baby, despite prior beliefs, Aki attempts to treat her cramps, despite the possible risk of not being able to have children. Kentaro decides to have an abortion, but needs consent from his "partner". He attempts to get in touch with Aki, whom he deduced is the mother, but she is busy and is further nervous around his boss who suspects something is wrong. Kentaro tells Aki about his pregnancy at her apartment. She is at first relieved that he wants an abortion, but a talk with a coworker convinces her the benefits of him having a child. When Kentaro tries to get her to sign the form, she tries to change his mind, but he refuses. Kentaro discovers that his meeting has been moved and Osugi has decided to give the project lead to Masato Tanabe who displays a slightly chauvinistic personality.

Kentaro hides his pregnancy so that he can prepare for his abortion. While sitting at the clinic, he encounters another man named Shota Miyaji who is also pregnant, with his second child (his wife had their first). While he befriends him, as well as his wife Noriko and their son Takuya, he tells him that he is considering abortion; leaving Shota visibly disappointed. While pitching UNIVE, Tanabe's presentation does not flatter investors. Kentaro suggests focusing on successful pregnant men, before revealing that he himself is pregnant. The idea is enthusiastically accepted by everyone. He tells Aki he is keeping the baby and she asks to help raise the child while also watching his diet. However, when Kentaro tells his mother, Tomoko, she is against it and he ignores her. Kentaro models for the company and becomes an instant celebrity overnight.

Kentaro starts to feel that his previous friends and acquaintances were never really close, but finds solace in Shota. The two of them decide to start an online community for pregnant men to meet various people in different situations. Aki reveals to one of her journalist friends that she is the mother of Kentaro's baby and he suggests doing a personal story about her life with him as opposed to an open-minded piece. Kentaro and Aki learn that they are having a boy and talk afterwards about the pros and cons of marriage. They soon learn that Shota had a miscarriage due to a cytomegalovirus infection. Noriko reveals that while she is sad, she is also relieved as Takuya was being bullied at school and people looked at her differently. Kentaro and Aki argue over the Miyajis' predicament, leading to Kentaro deciding to raise his baby alone.

Kentaro starts to become stressed out by work and tries to resign, but Osugi and the rest of the executives all coerce him into staying with the company. Meanwhile, Aki returns to her hometown to attend her sister's wedding. Her parents, particularly her father, demean her over the fact that her younger sister got married before her. She attends her high school reunion where she starts to get emotional when everyone reveals their shortsighted views on male pregnancy. She returns home and tells her parents her relationship with Kentaro before leaving them for good. She calls and makes amends with Kentaro and the two agree to stay together to raise their child, but not get married and have a "new kind of relationship". Kentaro is suddenly approached by a man, who claims that he is his father who also gave birth to him and whom Tomoko said was dead.

Tomoko confirms that the man, Eiichi, is Kentaro's father and he decides to let his father stay with him, despite Tomoko and Aki's concerns. Kentaro and Eiichi end up bonding with one another with the latter coming up with ideas for pregnant men products. Kentaro becomes invested in the ideas, but Aki thinks he is scamming him. Tomoko reveals to Kentaro that Eiichi's pregnancy was socially difficult for him and despite trying to have a decent life, it was too much for him and he ran away sometime after his birth. Aki's colleague offers her a job in Singapore, but she would have to live there for two years, thus she cannot bring herself to tell Kentaro whether she has accepted the position or not. In the morning, Kentaro is shocked to learn that Eiichi has been charged with fraud; putting his career and UNIVE in jeopardy.

Kentaro avoids the paparazzi before meeting with Shota where they make up. Later, he speaks with Aki and Tomoko and he realizes where his father may have gone. Following a comment from Kentaro, Aki and Tomoko have a heart to heart over what it means to be a parent. Kentaro finds Eiichi and the two resolve their issues. Kentaro returns home to find Aki who tells him that she will be taking the Singapore job, but that she will continue to be part of the baby's life and will be able to travel back and forth and provide for them. Kentaro's bosses set up a press conference to clear up the issue regarding Eiichi and gives an impassioned speech about roles which is viewed by everyone. Immediately afterwards, Kentaro suddenly goes into labor, while Aki, who was at the airport to fly to Singapore, rushes out to be by his side.