Jim lives a carefree life on Nandemoland, prioritizing play and operating the steam locomotive Emma with Lucas over formal education. His peaceful existence shatters when the demon Doringer attacks. Doringer's minion, Baron Bat, kidnaps Jim's mother Nina, a skilled dressmaker who creates enchanted clothing. Simultaneously, another minion, Black, uses transformation bullets: petrifying most adults and morphing the children, including Jim's friend Pokko, into animals—Pokko specifically becomes a small pink songbird. Jim narrowly avoids becoming a locomotive himself when one of Black's bullets accidentally hits Emma, paradoxically granting the locomotive sentience.

Possessing a crucial glowing button that amplifies its holder's abilities, Jim sets out on a quest. Accompanied by the sentient Emma and Pokko in her bird form, his goals are rescuing Nina, reversing the children's animal transformations, and confronting Doringer. Jim and Pokko share a bond of mutual affection, described as puppy love, which motivates Jim throughout their journey; Pokko's desire to regain her human form is a significant driving force. Pokko contributes practically by untying the string of a magical cloak Doringer forced Nina to create, neutralizing its defensive power and exposing Doringer's vulnerability.

Through these challenges, Jim's character develops from seeking personal rescues to actively combating Doringer's tyranny. He utilizes his magical button and relies on his companions Emma and Pokko to achieve his goals. Visually, Jim appears with only slightly darker skin compared to other characters in the anime adaptation, lacking clear racial or ethnic markers. This depiction differs significantly from other adaptations and the original German novels, which explicitly establish Jim as Black, linking his identity to themes opposing Nazi racial theories.