Tsuma, Shougakusei ni Naru.

Keisuke fell in love with Takae when they met through work. They got married and had a daughter, Mai, before Takae was tragically killed in an accident. Ten years later, an elementary school student appears on Keisuke's doorstep saying, "I am your wife!"

Keisuke and Mai are thrilled that Takae has returned to them, even if it is in the body of a grade schooler. They devote themselves to making up for lost time together. Meanwhile, Takae is dealing with family issues in her other life as a fourth grader.

Mai starts working at a new job but gets depressed after struggling to connect with her first client. To cheer her up, Takae invites her to see a fireworks show. Keisuke decides to stop by as well and happens to bump into his coworker Moriya on the way.

Mai is on her second date with Renji Aikawa, who then asks her to show him around Tokyo. Additionally, Chika, the mother of Marika (also known as Takae), has been seeing a man more frequently. Marika is now also supposed to get to know him.

Takae is nervous to tell Keisuke and Mai about Chika's plans to move and remarry, but the two take the news fairly well since the family will still be able to meet. Takae tries to stay positive about the move and hopes it will bring her mother happiness.

After seeing Marika (Takae) with Keisuke and Mai, Chika presses her daughter for an explanation. Takae's reluctance to respond sends Chika into a rage. Keisuke witnesses the outburst and starts trying to think of a way to help Takae leave her toxic home.

Mai and Keisuke receive a surprise visit from Renji. Keisuke is shocked to learn that Mai has a boyfriend and depressed to realize that she might eventually move out. Moriya notices Keisuke's mood at work and tries to think of a way to help cheer him up.

Chika and the Niijimas work together to support Marika after her memory loss. Mai tutors Marika to help her with school while Chika continues to work toward becoming a better mother. Meanwhile, Keisuke is worried that he may have lost his wife yet again.

Takae regains her memories and discovers the truth. Takae asks Chika to call her "Marika" to make things easier for when Marika eventually returns. When Keisuke hears about this, he suspects the same, but he still finds the idea very difficult to accept.

Keisuke explains the situation with his wife to Moriya, who is upset to hear that Keisuke wants to stay away from Takae for her own sake. Meanwhile, Takae joins Mai and Renji on a tour of the wedding venue and uses the occasion to make a special request.

Mai becomes very distraught when Keisuke tells her that he and Moriya plan to marry. Keisuke explains that Takae has merely possessed Marika and is eager to return the body to its owner. Mai refuses to accept this and turns to her mother for reassurance.

Mai takes Takae to the amusement park where Keisuke first ate one of Takae's bentos. On the Ferris wheel, Takae thanks Mai for her kindness and then notices Keisuke and Moriya walking below. Takae then witnesses as Keisuke makes a life-changing decision.