Enrico Bottini, a schoolboy in late 19th-century Turin, maintains a journal forming the core narrative. He belongs to a close-knit family with his father Alberto, mother, and younger brother Nino, who shares his all-boys school. Unlike Enrico's reserved nature, Alberto was mischievous and prone to pranks in his youth. Enrico begins with a kind heart and strong sense of justice.

The new school term brings change as beloved teacher Mrs. Delcati departs, replaced by the reputedly strict Mr. Perboni, initially dreaded by Enrico and his classmates. However, Mr. Perboni reveals himself as a gentle, reasonable mentor fostering empathy and rationality, earning Enrico's deep respect. Enrico's growth stems from community and school interactions. He befriends diverse classmates: dependable Garrone, bookish Stardi, and bullied Betti, demonstrating empathy during incidents like Betti's humiliation on parent-teacher day.

Enrico faces personal challenges and secrets, undertaking nightly work assisting his father with clerical tasks in the attic, a commitment hidden due to Alberto's pride. This teaches him diligence and familial responsibility. His development is further shaped by Mr. Perboni's moral stories exploring sacrifice, poverty, and compassion. A subplot involves Alberto reconnecting with his elderly, memory-impaired former teacher, Vincenzo Crosetti, who recognizes Alberto through Enrico's resemblance, reinforcing intergenerational bonds and education's value.

Throughout, Enrico matures emotionally and morally, learning to navigate complex social issues like bullying and class disparities guided by Mr. Perboni's humanist philosophy emphasizing inherent human goodness and mutual understanding. His journey culminates when his family relocates due to Alberto's employment change. Enrico experiences profound sadness, culminating in tears during a final picnic farewell with friends and teacher, signifying the depth of his connections and his coming-of-age transformation.