It's summer vacation and Tsuna is forced to take extra lessons due to his bad grades. After the teacher gives them homework, Takeshi suggests to Tsuna that they do it together to which Tsuna agrees. As the two decide on where to do the homework, Reborn arrives and tells them to do it at Tsuna’s home to which Takeshi happily agrees.
Later, Takeshi and Hayato arrive at Tsuna’s house. Tsuna is initially surprised to see Hayato but welcomes him when Takeshi tells him that Hayato will be able to help them. Takeshi is able to finish the questions with Hayato’s help except for question 7. Hayato looks at the question and is unable to understand it too. As the three try to solve it, Haru arrives carrying some food. Haru then leaves and Takeshi tells Tsuna that her uniform is that of the students of an elite school. Tsuna goes after Haru and asks her to look at the question.
After three hours of thinking, Haru tells them that she can’t solve it, stunning everyone. Just then, Lambo arrives, singing but after seeing Tsuna and Hayato’s faces, stops and gets something to eat. Takeshi suggests that they ask an adult and Haru calls a lady that she met. After hearing Bianchi’s name, Tsuna and Hayato panic and Hayato heads downstairs to lock the door. However, Bianchi easily melts the door and enters, causing Hayato to faint. After examining the question, she rips Tsuna’s paper, much to his dismay. As Tsuna panics, Haruyoshi Miura arrives and helps to solve the question too. After examining the question, he tells them that the answer is 3 but Reborn corrects him, saying that the answer is 4. After one look at Reborn, Haru’s father informs everyone that Reborn is the genius mathematician, Dr. Borin, to everyone’s surprise. However, Reborn ignores them and goes to sleep.
*Note: Bold + Italicized = First Appearance