Tsuna and his classmates are cleaning the swimming pool when their teacher announces that tomorrow the pool will open and any second-year boy who is unable to swim 15 meters will be forced to practice with the girls, causing Tsuna to panic. Yamamoto then arrives and offers to teach Tsuna to swim at the public pool which he accepts. Reborn then arrives and promises to help Tsuna too.
However, when they arrive at the pool Reborn's real reason for going with them is revealed: to relax in the pool and ignore his student. Tsuna then shows Yamamoto his swimming skill. However, he is only able to swim for about 5 meters. Yamamoto then gives Tsuna a demo of how to swim which he didn't understand. As they are about to try it, the two hears a voice asking for help. The voice turns out to belong to Haru who is pretending to drown in order to get Tsuna to rescue her. She explains to Tsuna that it is her method to help Tsuna learn to swim.
Yamamoto and Haru then continue to help Tsuna learn to swim. Gokudera then arrives and tries to teach Tsuna to swim too by explaining to him the theory. Since Gokudera, Yamamoto, and Haru have different ways of teaching Tsuna, they decide to hold a contest to see who will be able to get Tsuna to swim the farthest. However, all their methods fail to cause Tsuna to give up. As he tries to leave the pool, Reborn talks and tells him that he is lacking confidence and the three agree. Ryohei then arrives and tries to teach Tsuna to swim too. As he begins to argue with Gokudera as usual, Tsuna's leg seized up which causes Ryohei to jump to the pool. Everyone is surprised that Ryohei is unable to swim, but he thinks that he is a great swimmer and forgets to save Tsuna. After seeing Ryohei swim, Tsuna gains a little confidence and with the help and encouragement of his friends, he is able to swim 15 meters.
The next day, Tsuna and his classmates begin to swim. However, Tsuna is only able to swim freestyle while the test requires him to use breaststroke causing him to fail and practice with the girls...
Note: Bold + Italicized = First Appearance