"Monster Sonic! D'Artagnan's Rise to Fame" is an anime series centered around D'Artagnan, a young and ambitious monster trainer in a world where humans and monsters coexist. The narrative begins in the rural town of Freesia, where D'Artagnan dreams of becoming the greatest monster trainer in the world. His journey starts when he encounters a rare and powerful monster named Sonic, a lightning-fast creature with untapped potential. Together, they form a bond and set out to compete in the prestigious Monster Grand Prix, a global tournament that determines the top monster trainers.

As D'Artagnan and Sonic travel across diverse regions, they face numerous challenges, including rival trainers, mysterious organizations, and the complexities of monster training. Key rivals include the enigmatic Zephyr, a prodigy with a dark past, and Celeste, a charismatic but ruthless trainer who will stop at nothing to win. Along the way, D'Artagnan learns about the deeper connection between humans and monsters, uncovering secrets about Sonic's origins and the true nature of the Monster Grand Prix.

The series explores themes of perseverance, friendship, and the ethical responsibilities of training powerful creatures. D'Artagnan's growth as a trainer is paralleled by Sonic's evolution, as the monster unlocks new abilities and forms. The plot intensifies when a shadowy group, known as the Eclipse Syndicate, seeks to exploit monsters for their own gain, forcing D'Artagnan and his allies to confront them in a climactic battle that tests their resolve and teamwork.

Supporting characters include Mina, a skilled healer and strategist who joins D'Artagnan's team, and Rook, a veteran trainer who mentors D'Artagnan while grappling with his own regrets. The series balances action-packed monster battles with moments of introspection, delving into the motivations of its characters and the moral dilemmas they face. By the end of the series, D'Artagnan's journey culminates in a dramatic showdown at the Monster Grand Prix finals, where he must prove his worth as a trainer and protect the bond between humans and monsters.