My Stepmoms Daughter is My Ex begins with a scene where Yume and Mizuto become step-siblings after Yume’s mother Yuni marries Mizuto’s father Mineaki. Unfortunately, neither of their parents is aware that they dated in middle school. As a result, they agree that whenever either of them acts like an ex-lover, they must acknowledge that they are the younger sibling. Yume is duped into using Mizuto’s surname while they are attempting to provoke each other.
Several boys at school pretend to be Mizuto’s friends to get closer to Yume. Yume clings to Mizuto and claims she has a brother complex on his behalf, so the boys back off, though Yume is embarrassed her breasts made contact with Mizuno’s arm. As retaliation, she decides to sit next to him while wearing only a bath towel, tricking him into looking at her.
This nearly works, but she goes too far and her towel almost falls off until Mizuto assists her in remaining covered up. She tries to flee but trips and falls on the sofa with her. They forget about the game and almost kiss in this situation, with Yume almost naked, but are interrupted by their parents. They both recognize that situations like this are the reason they no longer like each other.
Kogure Kawanami approaches Mizuto, stating that he will be Mizuto’s friend and has no intention of using him to get to Yume. Akatsuki Minami excels at sports during fitness exams, while Yume overdoes it trying to keep up and faints, though Mizuto catches her. Yume requests that Minami keep how bad she is at sports
Yume is irritated by Mizuto, though Minami suspects he was trying to be nice.
Yume develops a fever, and Minami insists on visiting her. Yume is almost about to say something embarrassing to Mizuto when she notices Minami. Minami correctly deduces that Mizuto once had a girlfriend and wonders whether dating again will be awkward now that he lives with a stepsister. She also inquires whether he regards Yume as more than a sister, but he denies this.
Yume only falls asleep when Mizuto holds her hand, which he recalls they always did while they were dating if one of them was ill. After making several demands of him, Mizuto forces her to take her temperature again, revealing she had recovered but faked being ill so he would be nice to her. Mizuto forgives her after she apologizes.
Six months previously, while still dating, Yume obsessively kept every item Mizuto gave her. Today, Yume finds herself in a pair of Mizuto’s underwear. Certain that having them counts as losing their game, she is determined to return them. She attempts to sneak into his room, only to find Mizuto doing the same thing with her bra. Despite their suspicion, they both admit they broke the rules and can freely demand one thing.
The next day, Yume sees Mizuto with a girl and when he brings the girl home, Yume uses her one demand to force him to go food shopping, leaving the girl alone in his room, but she disappears. Mizuto demands Yume check her room, but she insists nothing has been moved or taken.
Mizuto is concerned because the girl was Minami in disguise, and she had requested that he marry her because she is obsessed with Yume, and marrying Mizuto would make Yume her sister. Minami’s childhood friend, Kawanami, admits Minami can become intensely obsessed. He tells Mizuto how to stop her as a favor. Mizuto later approaches Yume and, in response to his demand, asks her out on a date.
On the day of the date, Yume frets constantly over Mizuto’s motivation and for some reason finds herself extremely attracted to him. Via a flashback, it is shown the date was Kawanami’s idea. The date becomes tenser as Mizuto’s guilt mixes with Yume’s genuine awkwardness.
While Mizuto is in the bathroom, Yume becomes lost. Mizuto phones her but Yume finds she is disappointed when he is nice to her and does not mock her with sarcasm. She remembers a similar situation of being lost on their first date until he found her. Mizuto finds her and this time he mocks her, so they descend into their usual bickering and enjoy themselves.
Mizuto is surprised that they get along better as stepsiblings than as lovers.
Minami apologizes to Mizuto after watching the date and promises to control herself. She does, however, reveal that she is not yet ready to give up her Yume obsession. Yume, still perplexed by his request for a date, demands Mizuto submit to a punishment of sitting for a photo shoot, as she still finds him very attractive, and takes hundreds of photos
Yume asks Mizuto to help buy a gift for Yuni for Mother’s Day. As such, they buy roses, which makes Yuni cry tears of joy. Mizuto later prays at the shrine of his mother, who died when he was very young. As their parents are technically newlyweds, Mizuto suggests he and Yume give them a night alone by leaving the house. His suggestion of a cheap love hotel is rejected so Mizuto decides to stay overnight at Kawanami’s while Yume sleeps next door at Minami’s.
Kawanami admits to Mizuto that he and Minami were best friends once, but despite living next door they have drifted apart. The boys prank each other by yelling embarrassing things through the thin walls, making both girls mad at them. After dinner at a restaurant, Kawanami points out to Mizuto that while he and Minami argue openly about everything, Mizuto and Yume argue by trying to trick, undermine and outdo each other, which does not seem healthy.
Later that night, Mizuto and Yume meet on separate balconies. Mizuto comments that unless their parents have an unexpected divorce, they will be stuck as stepsiblings forever, which makes both of them sad.
Exams are approaching and Yume, as a top-ranked student, is determined to maintain her position and beat Mizuto. After the first exam, she is furious when she discovers Mizuto deliberately lowered his score to let her win. Yume starts to wonder if their rivalry was ever real.
However, after the exams are over, Yume discovers Mizuto has replaced her as a top-ranked student, though he challenges her to beat him during Finals if winning is so important. Yume realizes now she is second, she does not feel the pressure to be perfect and actually relaxes, which is what Mizuto had been trying to teach her. She rushes to find him to confess her love again but finds him talking to another girl, Isana Higashira, a severe introvert. Despite having met only recently, Mizuto considers Higashira his closest friend.
Yume tries to hide her jealousy, but Mizuto automatically knows what is happening. First, he tricks her into becoming flustered then demands to know what is wrong. Yume admits her jealousy so he antagonizes her so she returns to normal. After meeting Higashira, Yume is convinced they are just friends.
Yume and Minami interrogate Higashira over liking Mizuto. Minami declares she and Yume will help her get a date with Mizuto, but Higashira refuses as she prefers to just remain friends. Yume realizes Higashira is unsure of herself, just like she was in middle school. After watching Mizuto with Higashira at school, Yume and Minami are forced to admit that Mizuto is not romantically interested.
To kickstart some interest, Minami insists Higashira use her breasts to entice him. However, Mizuto fails to take an interest. Yume realizes trying to change Higashira’s personality to attract Mizuto is wrong. Minami decides to give her a makeover instead, but Mizuto barely reacts except to tease her.
As a last resort, Minami tells Higashira to confess as soon as possible. Higashira manages to give a heartfelt confession but Mizuto lets her down gently, explaining that while he is sure they would make a good couple, his heart already belongs to someone else. Yume is upset Mizuto still has such strong feelings he keeps hidden. Higashira returns to being Mizuto’s friend and finds she enjoys it more knowing there is no romantic tension.
Higashira asks to see Mizuto’s book collection, stating she would feel safe in his room since he already rejected her. Yume and Minami insist on updating Higashira’s clothing, still hoping Mizuto might show romantic interest. However, due to Higashira’s ample bust, everything she wears looks risqué.
Minami decides Higashira should dress like Yume, though to avoid looking like a copycat, Minami insists Yume also change clothes. Higashira finds a skirt she likes but Yume worries they have made Higashira too attractive. During the visit, Higashira is astonished at the size of Mizuto’s book collection.
Yume insists on joining them as a chaperone and is surprised to see them acting so casually. In private, Higashira admits to Yume she thinks she will always have feelings for Mizuto but will stay as friends to keep him in her life. With Higashira in the bathroom, Yume jealously asks Mizuto to put her socks on for her, but is embarrassed to find him touching her legs is erotic. Mizuto later wonders if he and Yume had remained friends instead of dating, would they now be as close as he is with Higashira?
In the past, Yume was nervous the first time she entered Mizuto’s room, thinking they might be ready to have sex, but all they did was read books. This upset Yume and made Mizuto angry at himself. Yume was further disappointed when she and Mizuto were placed in separate classrooms.
This forced separation allowed Yume to make her first friend. Unfortunately, Mizuto became jealous and snapped at Yume. When he tried to apologize, she accused him of cheating on her with another girl, resulting in them not speaking to each other. Weeks later, they attempted to reconcile but pride and embarrassment caused their relationship to stagnate to the point they did not speak for months and did not celebrate their first anniversary, Christmas, New Year’s Eve, or even Valentine’s Day.
Finally, on the day they graduated middle school, they broke up as Mizuto could no longer handle both loving and hating Yume at the same time. Two weeks later, their parents announced their engagement, and Yume and Mizuto became stepsiblings. Now in the present, during yet another argument, they both blame each other for not having sex the first time Yume visited Mizuto’s room.
Yume spots a cockroach in her room and insists on sharing Mizuto’s bed. Mizuto struggles with his feelings but does his best to forget them. While arguing the next day, their parents tease them. Mineaki announces they will be visiting his family for Obon, and as there will be swimming in the local river Yuni insists Yume and Mizuto buy swimsuits.
Yume is unsure about the suit she chooses and asks Mizuto’s opinion. He compliments her, but they are both embarrassed. Walking home, they pass the wall where they had their first kiss. Higashira later visits and Yume becomes jealous. Higashira is likewise jealous when Mizuto and Yume casually drink from the same cup.
Mizuto ends up falling asleep in Yume’s lap. Higashira is tempted to kiss him, though Yume dissuades her. Higashira then reveals she is tempted to try a friends-with-benefits relationship with Mizuto. Higashira leaves and though Yume is also tempted to kiss Mizuto, she also realizes forgetting her feelings would be for the best. Yuni is curious about Higashira and Mizuto and comments that Yume will probably find a boyfriend soon. However, Yume finds it impossible to picture dating anyone except Mizuto.
Everyone visits Mineaki’s family, which includes Mizuto’s second cousin, Madoka. Madoka senses something about Yume and Mizuto and reveals even as a child, Mizuto resisted becoming close to people. Later, Yume finds Mizuto in his great-grandfather’s office reading his autobiography detailing his time as a prisoner of war and meeting a Siberian woman called Elise. Mizuto suspects the book shaped his personality growing up, so he asks Yume to read it.
After reading the tragic love story, Yume begins to understand Mizuto. After she asks about Mizuto’s first love, Yume concludes it was Madoka. Madoka later asks if Yume is in love with Mizuto, but Yume is unsure. Madoka realizes Yume keeps finding excuses not to love Mizuto and decides to help by tricking them into spending time alone.
Having read the autobiography and met Mizuto’s family, Yume knows she has been selfish. Seeing her cry, Mizuto comforts her. She asks about Madoka but he denies ever loving Madoka and teases her for the mistake, causing her to laugh. As he leaves, Mizuto reiterates that his first love loved to laugh. Yume blushes at the implication she was Mizuto’s first love.
Everyone attends a fireworks festival. Mizuto takes pictures with Yume’s phone, somehow already knowing her password. Before the fireworks, Mizuto abruptly disappears. Madoka explains that every year, Mizuto watches the fireworks alone. Piecing together everything she has learned since becoming his stepsister, Yume realizes she never knew the real him.
Mizuto reflects on how books were the only thing that made him feel any emotion until he met Yume. Yume then finds him at the shrine and reveals her phone passcode was the date of their first kiss. When she asks why he decided to date her, he admits the truth. Yume kisses Mizuto and swears she will make him her boyfriend again, somehow.
Madoka is thrilled Yume made the first move. Returning home and to school, Mizuto finds that Higashira and Kawanami are competing to be his closest friend. Yume and Mizuto both separately conclude they must move on from their past relationship and begin a new one, though it is alright to stay as step-siblings for now. There the anime ends.
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