Ryûta Kondo, orphaned by the Hiroshima atomic bombing, initially survives by stealing rice. His resemblance to Gen Nakaoka's deceased brother Shinji leads to a mistaken identity. Upon explaining his situation, Kimie and Gen adopt Ryûta into their family.

Ryûta becomes Gen's close companion, helping him find work to support the family and buy milk for the malnourished infant Tomoko. They take a job caring for the severely burned survivor Seiji Yoshida; Ryûta starts rude but softens through their interactions. Though they earn money for milk, Tomoko dies of malnutrition before they return.

After Tomoko's death, Ryûta deepens his bond with Gen, formally calling him "older brother." He observes signs of recovery, including grass regrowing in the bombed area and Gen's hair growing back.

Three years post-bombing, Ryûta scavenges ruins with Gen for scrap metal and food. He joins Masa's gang of orphans, engaging in petty theft, black-market dealings, conflicts with rivals, and run-ins with law enforcement, marking a descent into delinquency.

Despite his rebellious activities, Ryûta remains loyal to his adoptive family. He collaborates with Gen and Masa to steal copper, attempting to buy penicillin for Kimie, who suffers from radiation sickness. After Kimie's death, Ryûta continues living with Gen and the orphan gang, helping build a communal home from salvaged materials—a physical reconstruction forming a new familial unit.

Ryûta's journey demonstrates how orphaned children regrouped as families in postwar Hiroshima, cycling through loss, adapting to harsh realities, and pursuing solidarity among those marginalized by the bombing's long-term effects.