Antonym is a slow-burning drama about human connection, told through the story of two women who come together to write a radio drama. Vain and self-centred Aya is forced to find a writing partner to work on the radio script she is developing. Heavily reluctant to collaborate, she partners shy co-worker Sachiko, who eagerly embraces the project. Peppered with coincidental occurrences and failed relationships, and powered by the friction and growing friendship between the women as they work on the script, Antonym is as much about the inevitability of human connection as it is about the necessity of it.

In the 17th century, two Portuguese Jesuit priests travel to Japan in an attempt to locate their mentor, who is rumored to have committed apostasy, and to propagate Catholicism.

At a train station in the suburbs of Tokyo, an accident takes place causing death and injury. Tsuyoshi is at the scene and witnesses the death of a woman. Right before the accident occurred, Tsuyoshi took a photograph of the woman, who carried a gentle look. Later, Tsuyoshi learns the woman has a daughter named Kiriko. Tsuyoshi meets Kiriko.

While inspecting a wharf, young Amano, president of an urban planning company, meets beautiful laborer Takako and falls in love at first sight. However, she does not return his affections, so he steals her ID card and goes on the run from the infuriated Takako.