"Nagagutsu Sanjūshi" is an anime series that adapts the classic tale of "The Three Musketeers" by Alexandre Dumas, reimagined with anthropomorphic animal characters. The story is set in a fictionalized version of 17th-century France, where the kingdom is under threat from political intrigue and external enemies. The protagonist, D'Artagnan, a young and ambitious cat from Gascony, travels to Paris with dreams of becoming one of the King's elite guards, the Musketeers. Upon arrival, he encounters Athos, Porthos, and Aramis, three skilled and loyal Musketeers who are also cats. Despite initial misunderstandings and conflicts, D'Artagnan earns their respect and joins their ranks.

The central plot revolves around the Musketeers' efforts to protect the kingdom from the schemes of Cardinal Richelieu, a cunning and power-hungry fox who seeks to undermine the monarchy. Richelieu manipulates events to create discord between France and England, aiming to consolidate his own power. The Musketeers, along with D'Artagnan, must navigate a web of espionage, betrayal, and danger to thwart Richelieu's plans. Key figures such as Queen Anne, who is a rabbit, and the Duke of Buckingham, a noble stag, play significant roles in the unfolding drama. The series also introduces Milady de Winter, a mysterious and dangerous feline agent working for Richelieu, whose complex relationship with Athos adds emotional depth to the narrative.

Throughout the series, the Musketeers face numerous challenges, including duels, ambushes, and political machinations. Their camaraderie and unwavering sense of justice drive them to overcome these obstacles. The story balances action, humor, and moments of heartfelt emotion, staying true to the spirit of the original novel while adding a unique twist with its animal characters. The series explores themes of loyalty, honor, and the struggle against corruption, making it a compelling adaptation of a timeless tale.