Kamerō Kameda navigates his second year of high school within an ordinary and routine existence. His true emotions and personal feelings remain deeply concealed, metaphorically hidden behind his ribs, reflecting his internalized nature and reluctance for open expression.

This quiet life fractures when his closest friend, Kaerugawa Erutaka, gains sudden prominence for assisting others. Kaerugawa's ascent propels him into the orbit of a prominent school group, including influential figures like Raionji Raiou, a standout student campaigning for student council presidency. Kaerugawa's abrupt integration into this elite circle triggers overwhelming envy and inadequacy in Kameda.

These consuming emotions dominate Kameda as he grapples with the shifting dynamics of their friendship. The culmination arrives during the student council election, marked by a pivotal "bone-to-bone collision." This confrontation signifies a profound turning point, implying both physical and metaphorical conflict intrinsically linked to the narrative's core theme of anatomical transformation mirroring personal change. The clash acts as a catalyst for Kameda's potential development, suggesting that altering one's fundamental structure—like bones—can forge a changed perspective or growth. His journey centers on this internal struggle and its external consequences within the school environment.