Hahi! Destroy Girls' Hearts! is the 4th episode of the Katekyo Hitman Reborn! anime.
The episode opens with a scene of Haru Miura running out of her house and walking on a fence. While on it, she bumps into Reborn and Tsuna while they are on their way to school. She is delighted to know that Reborn remembers her from before and asks him to be her friend. When he accepts, she jumps down acrobatically from the fence and asks him for a hug. He refuses and takes out the transformed Leon, saying that hitmen didn't perform those actions. At this, Haru hits Tsuna because she thinks that he is the one who influenced Reborn to say those things. While in anger at Tsuna, Lambo comes flying out of nowhere and lands on his face. As Haru comforts him, she notices Reborn and Tsuna running away, and says that they will continue this battle later.
In class, the teacher announces that Gokudera has gotten a perfect score on his math test, while Tsuna and Yamamoto have each scored 20%. On the way home, Yamamoto approaches Tsuna and asks if they want to do the homework together. Reborn pops out of a potted plant and invites him to Tsuna's house, saying that he had also invited Gokudera. When they walk inside, it is shown that Haru had followed them on the way back.
Inside, they have finished almost all the questions. Haru tries to go in, but is stopped by Nana Sawada before she can open the door. After saying that she was Reborn's friend, Nana welcomes her in. In Tsuna's room, they have finished the entire worksheet, with the exception of question seven. None of them are able to solve the question, but Yamamoto notices that Haru is wearing the uniform of Midori Middle School, an elite girls' school. However, Haru cannot solve the problem either, even after taking nearly the entire day. She tries to invite Bianchi in, but Gokudera prevents that from happening. Bianchi melts the door with her Poison Cooking, but instead of attempting to solve the problem, she rips Tsuna's paper to shreds. Haru then remembers that her father is a mathematics professor, and invites him to try the problem. However, he says that the question is unsolvable. Reborn then wakes up and says the correct answer, and Mr. Miura recognizes Reborn as Professor Boreen, a mathematics genius.
When Haru attempts to take Reborn home with her, he declares the "match" a tie. He states that he will stay in Tsuna's house until Tsuna is deemed acceptable as the Tenth. Later that night, while Haru is in bed, she rethinks Reborn's statement about the Mafia and decides that she still has to do something.
The next day, Tsuna is seen going to school, with someone in a ridiculous costume following him. His natural reaction is to run away, and the person follows him until the bridge. They take off their mask, and it is revealed to be none other than Haru. She says that if he can prove his strength, she will believe the Mafia story. She starts attempting to hit him with her hockey stick just when Yamamoto and Gokudera arrive. Gokudera throws his bombs because of the "threat to the Tenth," and when the smoke clears, Tsuna sees that Haru is gone. He sees her thrashing in the water below, and, after being shot by the Dying Will Bullet, dives into the water and saves her.
Later, on the shore, Haru is reprimanded for her actions. She imitates Tsuna's words from before just as Kyoko and Hana walk by. Haru declares her crush on Tsuna, and he attempts to scale the rock wall. Hibari walks by on the beach, calling the crowd an "eyesore."