Ten-year-old Willie Fiedler is considered strange by his family for his fascinations with dead birds, bugs, and dying fireflies he keeps in a Heinz jar. His older sister Roxie openly calls him 'Willie the Weirdo,' while their parents Sharon and Richard tolerate him. Willie spends most of his time in the bedroom of his adoptive grandfather, James Jonas Fiedler, an elderly man who barely speaks to anyone else but tells Willie elaborate, often impossible tales—claiming to have fought at Gettysburg, sailed with Cleopatra, and worked as a cop beating Black people in Selma.

Willie knows many of Grandfather's stories are lies but enjoys them anyway, especially gruesome details like the flag boy whose blood gleamed 'like rubies in the sun.' After being kicked by bullies, Willie returns home and listens to Grandfather describe the Yellowstone supervolcano that could cause an extinction event, both relishing the apocalyptic scenario.

That winter, the COVID-19 pandemic spreads worldwide, which Willie finds satisfying. Grandfather begins coughing blood and is diagnosed not with the virus but with pancreatic cancer that has spread to his lungs. He declines rapidly, becoming bedridden, jaundiced, and incontinent. Sharon cares for him reluctantly while Willie enjoys the smell of decay and continues their visits.

Willie repeatedly asks Grandfather about death, wanting to witness it and be the last thing Grandfather sees. One night, Grandfather signals that this is his 'big day' and beckons Willie close. With surprising strength, he grips Willie's neck and tells him 'You want death? Get a mouthful,' echoing Willie's own wish back at him. Moments later, Willie calmly enters the kitchen to tell Sharon and Roxie that Grandfather has died, stroking his philtrum—Grandfather's signature gesture—suggesting something has been transferred or passed on.