As the weekend approaches, Arthur and his friends discuss their plans. Richard excitedly reveals he's bought four tickets for a cosplay festival, but Henry declines, saying he has soccer practice. Arthur also declines, mentioning that he's invited Nora over to introduce her to his sister, Luna. This leaves only Richard and Jane interested in going. Arthur and Henry exchange knowing glances, realizing this is the perfect opportunity for Richard to spend some quality time with Jane and possibly confess his feelings for her. Jane, unaware of their intentions, asks Richard to join her since their other friends are busy. Richard agrees and hands the remaining two tickets to Arthur, suggesting he might use them with Luna or Nora.
After school, Arthur approaches Nora and gives her his home address before heading home. The next morning, Arthur wakes up early and takes a jog with Luna. After breakfast, Arthur hesitantly asks Luna if she's annoyed that he's asking her to meet Nora on her day off. Luna, however, is curious to meet the girl her little brother can't stop talking about.
Later, Arthur receives a call from Nora, who says there's only a gym at the address he gave her. Arthur hurries downstairs to find Nora standing beside her car, confused. He explains that the gym is theirs and that their home is above it, then takes her upstairs. Luna warmly greets Nora and apologizes for not informing her about their unique living situation.
As the three of them chat, Luna asks Nora about Arthur's behavior at school. Luna mentions that Arthur's kindness and caring nature, coupled with his cute face, attract a lot of girls, though he doesn't seem to notice. Nora agrees, sharing that Arthur's warmth helped her make friends and that her life has become much brighter since he entered it.
Wanting to continue her conversation with Nora, Luna challenges Arthur to a sparring match in the gym, with the loser having to cook lunch. Nora is appointed as the audience. Despite his best efforts, Arthur loses to Luna, who teases him about not being able to defeat her, even after she had taught him all the training she received from their late mother. At the mention of their mother, Nora inquires about their parents, and Luna reveals that they passed away in a car accident when Arthur was 14. Since then, Luna has taken care of him.
With Arthur in the kitchen, Luna shows Nora some photos, including the recent picture of Arthur pulling a cute face with puppy eyes. Luna asks Nora to keep it a secret, as she promised Arthur not to show it to anyone. Luna then reveals that her phone wallpaper is the same photo, explaining that seeing her little brother's face gives her the strength and motivation to protect his happiness. She offers to teach Nora some fighting techniques, but Nora politely declines.
Over lunch, Nora compliments Arthur's cooking, surprised that he knows how to cook. Arthur explains that he often has to fend for himself since Luna is busy with work. Misunderstanding the situation, Nora assumes Luna works multiple jobs to support Arthur's education and feels sorry for her. She offers to ask her father to give Luna a job at one of their companies, unaware that Luna is the CEO of Nexus and owns three luxury hotels.
Feeling it's better to come clean, Luna and Arthur reveal the truth about their wealth. Luna explains that they choose to live simply because Arthur wanted to stay in the house their mother once lived in. Nora is shocked to learn that Luna, one of the city's wealthiest individuals, leads such a modest life.
After lunch, Arthur suggests they all go to the cosplay festival with the tickets Richard gave him. Luna buys her own ticket since there are only two left. At the festival, they enjoy trying on different costumes, and Nora, initially hesitant, soon forgets her privileged background and starts having fun like everyone else.
As the evening falls, the three head to the bus station where Nora's car is supposed to pick her up. While waiting, Arthur tells Nora that Luna hasn't revealed her identity to anyone else, not even his close friends, which means she truly sees Nora as a friend. Luna sends Nora the pictures they took that day, telling her she's welcome at their home anytime and saving Nora's number as "Little Sis," which makes Nora happy.
As Nora's car arrives and she leaves, Arthur and Luna wait for their bus. In the car, Nora looks at the photo of her and Arthur dressed as a prince and princess, setting it as her wallpaper. She smiles, reflecting on how meeting Arthur and transferring to Sunrise High School has brought color into her once blank life.