Marcelino is an orphan boy left as a newborn on the doorstep of a Franciscan monastery, where the friars raise and educate him. He grows into a cheerful and mischievous child, living in close company with animals, trees, and plants, and possesses the ability to communicate with animals—a gift central to his many adventures.

Although cared for by the monks, he nurtures a deep curiosity about his unknown mother and embarks on a quest to find her. This leads him to partner with a gypsy girl named Candela, who holds unexpected knowledge of his origins. In a significant event, Marcelino discovers a wooden Christ figure in an attic and offers it bread and wine; the figure miraculously consumes them and grants his wish, resulting in a reunion with his deceased mother.

After this supernatural encounter and a journey into the afterlife, he chooses to return to the human world to continue his adventures. His return enables further mischief, discoveries, and interactions with a variety of characters, including friends like Candela and Leo, and adversaries such as Duke Arthur Mostro and the hunter Lucas. Marcelino takes delight in uncovering new things and frequently poses unusual questions, which propel many of his stories.