"Fate/Grand Order: Fujimaru Ritsuka wa Wakaranai Dai Bōnen O-Tanoshimi Kai" is a comedic spin-off series set within the expansive "Fate/Grand Order" universe. The narrative centers on Fujimaru Ritsuka, a Master of Chaldea, an organization dedicated to preserving human history. The series diverges from the main storyline’s high-stakes battles and instead focuses on the daily lives and humorous interactions of the Servants and staff at Chaldea.

The plot revolves around Ritsuka’s attempts to organize a New Year’s celebration for the Servants, who come from various historical and mythological backgrounds. Each episode features lighthearted vignettes showcasing the eccentric personalities of the Servants as they prepare for the festivities. Characters like the mischievous Gilgamesh, the diligent Mash Kyrielight, and the ever-competitive Jeanne d’Arc and Altria Pendragon (Saber) contribute to the comedic chaos. The series highlights their quirks and relationships, often resulting in absurd and entertaining situations.

The humor is derived from the clash of personalities and the anachronistic nature of the Servants’ interactions, as well as their attempts to adapt to modern customs. The series maintains a light tone, with no overarching conflict or dramatic stakes, focusing instead on slice-of-life moments and character-driven comedy. It serves as a playful companion to the main "Fate/Grand Order" narrative, offering fans a chance to see their favorite characters in a more relaxed and humorous setting.