"Eiga Oshiri Tantei Saraba Itoshiki Aibō (Oshiri) yo" is a feature-length anime film based on the popular children's book series "Oshiri Tantei" by Troll. The story centers around the titular character, Oshiri Tantei (Detective Buttocks), a quirky and endearing detective with a literal buttocks-shaped head. Known for solving peculiar cases with his sharp intellect and unconventional methods, Oshiri Tantei is accompanied by his loyal assistant, Brown, a small dog with a knack for providing comic relief and moral support.

The narrative begins with Oshiri Tantei and Brown receiving a mysterious letter from a long-lost friend, prompting them to embark on a journey to uncover the truth behind the message. Along the way, they encounter a series of bizarre and humorous mysteries, each requiring the detective's unique problem-solving skills. The film weaves together multiple episodic cases, blending lighthearted humor with moments of suspense and adventure. The cases often involve eccentric characters and absurd situations, staying true to the whimsical tone of the original book series.

As the story progresses, the central mystery deepens, revealing a connection to Oshiri Tantei's past and his relationship with his former partner. The emotional core of the film lies in the themes of friendship, loyalty, and the importance of perseverance in the face of challenges. The climax brings together all the narrative threads, culminating in a heartfelt resolution that underscores the bond between Oshiri Tantei and Brown.

The film maintains a playful and accessible tone, making it suitable for younger audiences while retaining enough cleverness to engage older viewers. The characters are distinct and memorable, with Oshiri Tantei's unwavering determination and Brown's unwavering support serving as the driving force behind the story. The episodic structure of the cases keeps the pacing dynamic, ensuring a balance between humor, mystery, and emotional depth. Overall, "Eiga Oshiri Tantei Saraba Itoshiki Aibō (Oshiri) yo" is a charming and entertaining addition to the Oshiri Tantei franchise, offering a mix of laughter, intrigue, and heartfelt moments.