Miki Koishikawa lived a normal life - until her parents tell her they'll get divorced.. She doesn't agree, until she meets Yuu Matsuura... the handsome son of another couple, with whom her parents are switching spouses. They all live together now in an enormous house, and Miki and Yuu find themselves falling in love. They'll have to struggle with their own feelings, other people who want to be with them, and whatever Lady Luck and Fate throws in their ways...
While Ginta is pestering Miki to tell him how she feels they get interrupted by none other than, Yuu. Miki runs in the house and Ginta learns about how Miki and Yuu really know each other. Later the next day Miki tells Ginta that she's really confused right now, and until she sorts out how she feels they should continue being friends. After school that day Meiko and Miki find out where Yuu works and they meet his boss. He tells them his amazingly beautiful story about him and his current wife but doesn't tell them that it's him. The next day Ginta is practicing and find out that his partner for the tournament has broken something and has to take it easy. Yuu is asked by Miki to be Ginta's partner and he accepts it. Yuu agrees and runs to the library and he sees something happen between Shinichi and Meiko. What did he see?
Miki's class is going on their trip to Hokkaido. Miki and Meiko are in the same group and Yuu and Ginta are in the same group. Miki's group goes on their tour; they star with the tower in Hokkaido. Miki takes a second to see the gift shop; she turns around and they're gone. She goes and looks for them and finds Yuu and Ginta instead. Then Arimi randomly comes and grabs Ginta's arm. Does this bother Miki?
With the whole fiasco with Meiko Shinichi decides to quit as a teacher and work as a real estate agent with his parents in Hiroshima. He tells the class this with Meiko absent she has no idea on what is going on with him. When Miki wants to talk to her Meiko says something that upsets Miki a lot. What happens next?
Miki and Yuu's parents go for a trip, leaving Miki alone with Yuu. Yuu is talking a bath, while Miki is in her room waiting for the bath and writing to Meiko. Yuu comes in her room telling her that he's done giving her her turn. She takes a nice hot bath and apparently takes too long. She wraps the towel around herself and then faints. Yuu notices and runs down stairs. What does he do?
Meiko is thinking about tell Miki that Shinichi and She are together but she can’t seem to bring herself to say it. The next day the tournament was stating with the match against Yuu’s old school. This guy there that is related to Ginta and Yuu (in different ways) is their opponent. His name is, Tsutomu Rocktanda, and he is Ginta’s cousin. He knows about Miki’s letter to Ginta and laughs about it. Miki all mad tells Ginta “I hate you” and runs off. This affects Ginta’s game. Does Miki come back to cheer them on, or do they lose the tournament?
Miki is still trying to figure out who she likes, Yuu or Ginta. The end of the school day Yuu asks Miki for her help at the shop just for the day. Ginta walks by and sees them talking but is wondering what they're saying. Later that day Ginta and Arimi drop by the shop for their plan. That's when Tsutomu comes in. He challenges both; Ginta and Yuu, with the cost for the loser will shave their head. Who wins and who wins? And what happens when Yuu and Miki get home?
It's a new day and Miki's looking at the pictures that she went to hokkaido. She keeps thinking of weather she's jealous or not. She wants to stay in bed and avoid seeing Yuu, Though he won't stop knocking on the door till she gets up. She comes down and there's a BBQ going on in the backyard. It's a family welcome back party for Yuu and, Miki. Miki has realized that she's getting caught up in the atmospere; though she's still doing her best to fight it. Her father suggests they all go out to have some fun together, everyone says yes except one person, Miki. She runs off and goes to a shop where she runs into, Arimi. When she gets home a vase breaks and it looks like they had a fight. What happens to the 6 of them?
Miki swears that she won't forgive Ginta for kissing her since they weren't in love. The next day, she tries to get up late so she won't meet Ginta but Yuu forces her out of bed. When Yuu and Miki arrive at school and met with Ginta and Meiko, Miki just hands Meiko her diary and leaves, Ginta angrily walks the other way. Meiko understood the story from Miki's diary. After school, Miki makes Meiko take her place as cleanup student and when Ginta calls her, Miki warned him to stay out of her sight before running off.
Miki has told Meiko about the photo she discovered, and in her reply, Meiko said, ""If you're so concerned about it, then that means......YOU'RE IN LOVE WITH HIM!"" The next day, Yuu is at the library reading, when a good looking Senior Student comes in and sits next to him; it wasn't long before Yuu realized that the Student was staring at him in an admiring look, Yuu askes him why he was staring at him, the Student replies, ""You're interesting to look at."" Yuu gets a little scared and runs off. That night, Chiyako gives Yuu and Miki some free tickets to an amusement park called ""Wonder Dog"". Miki was excited, and when Yuu says he'll acompany her, she starts to think this could be a date.
Miki is at her crazy house getting ready for school and her parents all left. Only her and Yuu remain in the house. She's been trying to avoid Yuu since ""the kiss."" But she can no longer avoid him and goes into the kitchen (where he is). This episode is where you get the name of the series 'Marmalade Boy!' Miki compares Yuu to marmalade saying he has bitter bits inside him but everyone sees his sweet looking outside: Marmalade Boy! And Yuu compared Miki to mustard: always sour and spicey, Mustard Girl! Well, Miki's strange family seems to get along together (though it is rather strange...).
Miki is getting jealous because everyone at the school is under the impression that Yuu and Satoshi are a couple. Since Summer Vacation Started Yuu has been working all day and comes home really tired. Miki doesn't have a chance to talk to him anymore. When Miki and Meiko are out shopping, Meiko has the idea that they should go to Yuu's store and she should buy that dress that she's been needing. The trip also has the ulterior motive to find out the real relationship between Yuu and Satoshi, who just happens to be at the store. Will Miki Get to the bottom of this rumor that is going around school?
The episode begins with Miki saying that her life is too boring and she wishes it would be more exciting. And she kinda got her wish... Her parents were getting divorced. But this was a very strange divorce. They were just gonna switch partners with another family, the Matsuuras. During their romantic trip to Hawaii, they met the Matsuuras. Miki's dad fell in love with Mrs. Chiyako Matsuura and Miki's mom fell in love with Mr. Youji Matsuura. So they talked about it and decided to switch partners.
Miki's in her room studying for final exams, when she is called for dinner. Since after the exams schools out the family will be going to register their marriages and then going to eat at an Italian restaurant for a celebration dinner. After the registration they were on their way to the restaurant to eat, but suddenly Miki is feeling weird and decides to take a rain check for it. After leaving she keeps thinking about when she was little and what was going on with her parents. She takes a break and sits down. After she sits Yuu decides to say that he's there. Later they go for burgers and, rubs it in Miki's face that he gave up the gourmet meal. After getting home she goes to her room. She replaces the picture in her medallion from her and her parents to Yuu's picture. The next day someone with a camera that goes to Yuu and Miki's school is video taping everyone that come out of the house. He's video taping them for a story. How does this video come out?
Miki walks into her room drying her hair and thinking about the day before. The next day she and Ginta go to the roof to talk. She tells him the truth about her feelings and that she likes both Yuu and Ginta. She tells him that he should date Arimi instead. After school that day Miki and Meiko go to the coffee shop and talk. Meiko say something that confuses Miki even more. Later that day she goes shopping and bumps into Arimi on the sidewalk. She makes a little joke that Miki takes seriously. She's on her way home and sees Ginta in front of her house and he confesses everything and the truth behind his relationship between him and Arimi. What else happens between them?