"Eiga Oshiri Tantei Nandemo Kaiketsu Club tai Kaitō U" is a feature-length anime film that expands on the popular "Oshiri Tantei" (Butt Detective) series, known for its lighthearted and comedic approach to mystery-solving. The story centers on the titular character, Oshiri Tantei, a detective with a literal butt for a face, who solves cases with his sharp intellect and quirky charm. Alongside his loyal assistant, Brown, and other recurring characters, Oshiri Tantei faces off against a new adversary, the enigmatic thief Kaitō U.

The narrative begins with the Oshiri Tantei Nandemo Kaiketsu Club, a group dedicated to solving any mystery, receiving a challenge from Kaitō U, a master thief who prides himself on his ability to steal without leaving a trace. Kaitō U announces his intention to steal a priceless artifact, setting the stage for a high-stakes game of cat and mouse. The club, led by Oshiri Tantei, must unravel a series of cryptic clues and outsmart the cunning thief before he can execute his plan.

As the story progresses, the club members work together to decode Kaitō U’s riddles, which lead them through a variety of whimsical and imaginative settings. Each clue brings them closer to uncovering the thief’s identity and motives, while also highlighting the unique skills and personalities of the club members. The film maintains a playful tone, blending humor with moments of suspense as the characters navigate puzzles and obstacles.

Kaitō U’s character is portrayed as a charismatic and elusive figure, adding an element of intrigue to the story. His interactions with Oshiri Tantei and the club are marked by a mix of rivalry and mutual respect, creating a dynamic that drives the narrative forward. The film’s climax features a dramatic confrontation between Oshiri Tantei and Kaitō U, culminating in a resolution that underscores the themes of teamwork, perseverance, and clever problem-solving.

Throughout the film, the story emphasizes the importance of collaboration and creative thinking, as the club members pool their resources and talents to overcome challenges. The lighthearted tone and accessible humor make it suitable for a younger audience, while the engaging mystery elements provide broader appeal. The film stays true to the spirit of the "Oshiri Tantei" series, offering a fun and entertaining adventure that builds on the established world and characters.