"Humanity Has Declined" is set in a post-apocalyptic future where human civilization has drastically declined, and the population has dwindled to a fraction of its former size. The story centers on a nameless protagonist, referred to as the Mediator, a young woman tasked with bridging the gap between humans and a new dominant species known as the Fairies. These Fairies are small, highly intelligent, and whimsical beings who possess advanced technology and a penchant for creating bizarre and often nonsensical inventions. Despite their childlike appearance and behavior, the Fairies are far more capable than humans in many respects, leading to a dynamic where humanity is no longer the dominant force on Earth.

The narrative unfolds in a non-linear fashion, with each episode or arc presenting self-contained stories that explore the interactions between humans and Fairies. The Mediator, often accompanied by her grandfather, navigates a world where the remnants of human society struggle to maintain relevance. Her role involves negotiating with the Fairies, solving problems caused by their eccentricities, and occasionally uncovering the darker truths behind their seemingly innocent actions. The series blends elements of satire, dark humor, and surrealism, often using the Fairies' antics to comment on human nature, societal structures, and the consequences of technological advancement.

Key characters include the Mediator, who serves as the grounded, pragmatic voice amidst the chaos, and her grandfather, a resourceful and eccentric inventor who provides support and comic relief. The Fairies, while collectively a central force, are portrayed as a unified yet enigmatic entity, often acting as both allies and antagonists depending on the situation. The series also features occasional human characters, such as the Mediator's classmates and other survivors, who provide additional perspectives on the world's state.

Themes of decay, survival, and the absurdity of existence are woven throughout the narrative, with the Fairies serving as a mirror to humanity's flaws and aspirations. The story's tone shifts between lighthearted comedy and unsettling darkness, creating a unique atmosphere that challenges conventional storytelling. "Humanity Has Declined" is a thought-provoking exploration of a world where humanity's decline is juxtaposed with the rise of a new, unpredictable order, offering a blend of wit, irony, and philosophical undertones.