On a summer day, Tsuna decides to head to his room to cool off. He finds Lambo sleeping, Reborn relaxing in the summer heat and Bianchi, attempting to poison him again. Reborn informs Tsuna that Bianchi will oversee Tsuna's tutoring as well. Tsuna, afraid for his life, tries to reason with Reborn but is ignored. Gokudera arrives, offering some watermelon to Tsuna. Gokudera, noticing something is wrong, offers to take care of Tsuna's trouble, but is shocked to discover that the problem is Bianchi, his sister. Gokudera excuses himself and runs out clutching his stomach.
After learning that Gokudera and Bianchi are siblings, Tsuna follows Gokudera and sees him supporting himself on a tree. Gokudera describes how in the past, he was forced to eat Bianchi's Poison Cooking before his piano performances; this led to stomachaches at the mere sight of her. Hoping to drive Bianchi out of town, Gokudera reveals a plan to find a lookalike of Bianchi's ex-boyfriend and have him lure Bianchi away. Tsuna notes that the picture of Bianchi's ex-boyfriend looks exactly like Teen Lambo. Tsuna confronts Lambo and asks him to use the Ten Year Bazooka, but Lambo refuses, saying he never fired it and is not allowed to. Tsuna seeks out Reborn and asks him to beat up Lambo but Reborn states he doesn't deal with opponents lower than him. Lambo comes out of his own will and accidentally electrocutes himself with his stun gun. Frustrated with his failure, Lambo uses his Ten-Year Bazooka on himself.
Tsuna calls Bianchi and she appears, mistaking Teen Lambo for Romeo. She rushes towards Lambo with a seemingly happy demeanor but abruptly smashes a dish of Poison Cooking: Two in his face. Reborn surmises that Bianchi and Romeo had a horrible relationship and the memory of it upsets Bianchi. Later, Tsuna meets up with Gokudera and tells him about what happened. Gokudera recalls hearing that Romeo died due to food poisoning. Meanwhile, Reborn comments that he wants to eat an eel on the hottest day of the summer. Subsequently, Bianchi bikes to Hamana Lake to fulfill Reborn's request, temporarily leaving town.
Note: Bold + Italicized = First Appearance