Bill is a man whose life is portrayed through an innovative use of split-screen windows, with narration provided (by Don Hertzfeldt, uncredited). Struggling with an unnamed illness, Bill's existence appears to be mundane until the reality of his deteriorating health begins to unravel. During routine meetings with his ex-girlfriend, he confronts the stark reality of his condition; during a visit to the doctor, he learns that his illness is progressively worsening. As the days unfold, Bill's hallucinations intensify, leading to a mental breakdown that culminates in him fainting in an alley.
In an effort to assist him in his recovery, Bill's mother arrives to care for him. However, a moment of confusion leads Bill to mistakenly attack her, believing she intends him harm. Subsequently, he is hospitalized, where his fluctuating health perplexes his doctor. In a surprising twist, the medical team concludes that Bill will not pass away, a revelation that creates discomfort among his family members. The very next day, Bill attempts to return to work.
The narrative also delves into Bill's childhood, as the Narrator recounts the tragic story of his half-brother, Randall, who met an untimely death while chasing a bird into the sea. Following this incident, Bill’s mother became excessively protective, rarely allowing Bill outside and eventually driving his stepfather away. The Narrator elaborates on the surreal and often tragic lineage of Bill's family, many of whom battled mental illness and faced grim ends.
After a brief spell out of the hospital, Bill receives a call about his mother’s passing, attributed to a "fit of senile hysterics." Following her funeral, he discovers a notebook filled with love notes intended for him during his childhood. In an unexpected follow-up appointment, Bill learns from his doctor that there appears to be nothing physically wrong. However, while out for lunch, he suffers a seizure, causing a rush of fragmented memories from his early years to flood back.
Bill finds himself back in the hospital once more, with regular visits from his ex-girlfriend. A new doctor probes into Bill's past, exposing a troubling inability to recall basic details of his life. After undergoing a brain examination filled with odd and nonsensical questions, Bill's doctor informs him that he's struggling to differentiate between past and present. It’s implied that many of his childhood memories may be fabrications.
After being discharged to receive familial care, Bill returns home only to find it deserted. As he grapples with his memory loss, he begins to forget routine tasks like grocery shopping or taking walks, remaining oblivious to his condition. Ultimately, his doctor breaks the news that Bill’s time is limited.
This harrowing revelation prompts a profound transformation in Bill, as he begins to observe the small, often overlooked details of life. The animation style shifts dramatically in tune with his changing perspective, incorporating vibrant, full-color photography into the scenes. Acting on instinct, Bill rents a car to visit his childhood home. There, he reconnects with his uncle, who provides him the address of a nursing home where he can meet his estranged father, a man he has not seen since his youth. After reconciling with his father and offering forgiveness, Bill continues his journey.
Feeling increasingly unwell, he takes the time to rest beneath a tree, and the screen fades to black. In an imaginative twist, the Narrator proposes an alternate fate for Bill, one where he achieves immortality, engaging in remarkable feats, ultimately outlasting the human race, witnessing the slow, cosmic twilight of the universe as stars gradually fade into darkness.
