After the defeat of Mukuro, life goes on normally and Tsuna and his friends all move up a grade at school. However, none of them are in the same class, so Reborn offers them a challenge to catch him, with the prize being a change of the roster. Later, they celebrate at a sushi restaurant owned by Yamamoto's father only to find they do not have enough money to pay for their food and have to work off their debt.

A burglar comes into Tsuna's room at night, with Tsuna fainting from fear, only to wake up in the morning to find him dead. Everyone must find a way to hide the body or else go to prison.

Kyoko's friend Hana is infatuated with Future Lambo and blackmails Tsuna into letting her meet him by threatening to tell Kyoko that Haru is his girlfriend. When Lambo uses the Ten-Year Bazooka, however, something malfunctions and creates a teen Lambo with young Lambo's mind, and to make matters worse, this Lambo escapes from the house.

Reborn invites Tsuna and his mom to a vacation resort, but they first have to ride on a cruise ship to get there. Once on the ship, Tsuna spots some of his friends, who all boarded without tickets, and he now has to keep them from being caught, but first, he has to catch them.

At Mafia Island, Tsuna fails to prove his identity as the Vongola Family's 10th boss, and is transported to the back of the island where he is given a new chance to prove himself. There, he meets another Arcobaleno, Colonello. Colonello and Reborn put Tsuna through tough training. Meanwhile, another Arcobaleno invades the island with an army.

Tsuna has to swim 15m in school, but being unable to swim, he is trained by Yamamoto, Gokudera and Haru in the public pool. Everyone has his own method, and they decide to compete to see who can get Tsuna to swim the furthest. Reborn interferes this time with a Vongola dolphin that has a dangerous complexion.

To pay off the damage caused in the previous episode, Tsuna works as a lifeguard with Ryohei, only for them and the other boys to be challenged to a swimming race by older lifeguards. Later, Tsuna and his friends run a stand during a summer festival to continue to pay off their debt, only to have run ins with Hibari, and a group of thieves.