"Bottom Biting Bug" is a surreal and darkly comedic anime series that explores the bizarre and unsettling experiences of its protagonist, a high school student named Taro. The story begins when Taro encounters a strange, insect-like creature that bites him on the lower back, an event that triggers a series of increasingly absurd and disturbing events. The bite marks begin to multiply, spreading across his body, and Taro soon discovers that the marks are linked to a mysterious phenomenon affecting others in his town. People who are bitten exhibit erratic behavior, often leading to chaotic and unpredictable outcomes.

As Taro investigates the source of the bites, he meets a group of individuals who have also been affected, each dealing with their own unique and often grotesque transformations. Among them is Aya, a classmate who develops an unusual connection to the bugs, and Kenta, a former athlete whose physical abilities are amplified in strange and uncontrollable ways. Together, they uncover the existence of a hidden underground ecosystem where the bugs originate, governed by a cryptic entity known only as the "Queen." The Queen appears to manipulate the bugs and their effects on humans, though her motives remain unclear.

The narrative delves into themes of alienation, bodily autonomy, and the fragility of human identity as Taro and his companions struggle to understand their condition and resist the Queen's influence. The series balances its dark subject matter with moments of absurd humor, often juxtaposing the grotesque with the mundane. As the story progresses, Taro's journey becomes increasingly introspective, questioning the nature of free will and the boundaries between humanity and the natural world. The series concludes with an ambiguous yet thought-provoking resolution, leaving the fate of the characters and the bugs open to interpretation.