The first book in the Lotterys series introduces the Lottery family, a large, unconventional household in a sprawling Victorian home called Camelottery in Toronto. Four parents form two married couples who are also best friends: PopCorn (born Reginald Miller, from Yukon) and PapaDum (from Delhi, the oldest parent at 59); CardaMom (Mary Johnson, a Mohawk former lawyer) and MaxiMum (from Jamaica). After winning the lottery, they bought the house and homeschool their seven children, all named after trees. Nine-year-old Sumac, the fifth child and the story's protagonist, is a precise and responsible girl who serves as the family's organizer.
One July morning, PopCorn receives a phone call reporting that his 82-year-old father, Iain Miller, has set his house on fire in Faro, Yukon, by leaving sausages in a deep fryer while taking a bath. PopCorn must fly out immediately. The children learn that this "fourth grandfather" is essentially unknown to them: He has never met any of his grandchildren, and PopCorn visits him only rarely. When Sumac presses for an explanation, PopCorn gives evasive answers. Sumac's planned week-long study of ancient Mesopotamia with PopCorn is canceled, and she insists on accompanying him, arguing that two visitors will cheer Iain more effectively than one.
In Faro, a town of about 400 people, Sumac meets Iain for the first time. He is tall and bony, balding with gray hair, his eyebrows burned off and his hands bandaged. He stares at Sumac without greeting her and mishears her name as "Smack." At a nursing center, Sumac tries to show him photos of his grandchildren, but he responds dismissively and reacts with hostility when reminded that PopCorn changed his surname to Lottery. Only after a failed gold-panning outing does PopCorn explain that his father disapproves of his marriage to a man. PopCorn spends most of the trip arranging care by phone, and Sumac overhears a tense Skype call in which he discusses a cognitive assessment that counts as mild impairment but will only worsen and proposes bringing Iain to Toronto. CardaMom insists Iain is family; MaxiMum responds with calm skepticism. PopCorn tells Sumac they are bringing his father home for a visit.
Iain's arrival at Camelottery is turbulent. Brian, the four-year-old who chose her own name and refuses to be called a girl, asks bluntly where his eyebrows are. Iain insists his son is "Reginald" and recoils from the pet rat belonging to Aspen, Sumac's 10-year-old sister. The parents study online materials about dementia caregiving, but the advice is discouraging: The ideal environment is small, modern, and single-level, nothing like Camelottery. The children struggle to find a name for Iain; Aspen coins "Grumps," which sticks as a private nickname.
Based on CardaMom's suggestion that people with dementia prefer discussing the past, Sumac creates a slideshow of old family photos. At dinner on the back porch, Grumps watches a few images of his late wife, then shoves back his chair and storms inside. Sumac is devastated. The parents reveal that Grumps's stay may be permanent rather than temporary.
When the three eldest siblings, Sic (16), Catalpa (14), and Wood (12), return from Camp Jagged Falls, the parents announce at a local pizzeria that Grumps will be living with them "for the moment." The children react with dismay. The next morning, MaxiMum explains that Grumps needs a ground-floor room, which means Sumac must give up her bedroom. Furious but unwilling to seem ungrateful, Sumac moves her belongings to a cramped attic room alone. CardaMom praises Sumac's generosity publicly, making it impossible for her to complain.
MaxiMum asks Sumac to serve as Grumps's guide, showing him around and explaining how the family does things. Sumac never agrees but cannot refuse. Every interaction proves difficult. Grumps dismisses the family's environmental practices, uses racist language toward Sumac and MaxiMum, and calls homeschooling nonsense. In his room, Sumac discovers a calendar with slashes through each day and a circle around July 31, his intended departure date. He insists he is "compos mentis," or of sound mind.
The Lotterys repeatedly change their plans to accommodate Grumps. They skip a Powwow, a Native community gathering where they would have visited Baba, CardaMom's father, and attend a dull church service instead of a Bahá'í meeting. None of it makes Grumps happy. At breakfast, he declares that wolves pair one male with one female for life, a pointed rejection of the household's same-sex parents. At the beach, his remark that children in his day simply jumped in triggers Brian's refusal to wear a life jacket; MaxiMum stays closely beside Brian, stressing physical confidence and lifting her when needed. When Grumps swims far out in the lake, a couple returns him to shore, reporting he seemed disoriented. During a chess game, he accuses Sumac of cheating for attempting to castle, a legal move he no longer recognizes. His struggle to produce an insult, stumbling over fragments before landing on "ignoble," is itself a sign of decline.
On July 30, the parents reveal that Grumps's test results confirm significant cognitive deficits. He cannot return to Faro. When PopCorn and PapaDum explain this, Grumps erupts, calling them robbers and abductors. Sumac devises a plan to nudge Grumps into wanting to move to an assisted living facility. She researches retirement homes and enlists Aspen, though Aspen's sabotage attempts are counterproductive. Meanwhile, other family members warm to Grumps. He teaches Brian about different types of marbles, and they play together using a polystyrene cup. Wood bonds with him over the natural world.
At a toy store, Brian steals marbles to replace the ones Grumps has "lost," misunderstanding the dementia metaphor literally. When caught, Sumac blurts out an explanation of Grumps's condition, humiliating everyone. That evening, Sumac delivers a researched presentation about Sunset Vista Residence, a retirement home with a pool and round-the-clock nursing. Sic shuts the laptop. Grumps declares the family wants to be rid of him and retreats to his room. PopCorn says flatly that Grumps may need to feel wanted, even when he says he wants to leave. Sic comforts Sumac afterward, framing their obligation as a chain of loyalty: PopCorn owes his father, and the family owes PopCorn.
On Oak's second birthday, the children play Blind Man's Buff with Grumps, who suggested the game. Oak, the nearly-two-year-old whom the parents describe as different and on his own developmental track, crawls unnoticed into the room. Blindfolded, Grumps lurches forward and his boot comes down on Oak's hand, breaking a finger. Sumac scoops up Oak and tells the parents that Grumps "stamped on Oak's hand" without mentioning the blindfold, making the incident sound deliberate.
The next afternoon, Sic discovers Grumps has disappeared. Sumac guesses the airport. The family races to the terminal, where Sumac spots Grumps alone against a plate-glass wall. He calls her "Sue," and she gently corrects him. He tells her his credit card was canceled and he could not buy a ticket. Sumac tells him, "We want you to stay." At a family meeting on the backyard trampoline, the parents announce they plan to move Grumps to Sunset Vista. Sumac confesses her lie: Grumps was blindfolded, and stepping on Oak was an accident. She argues fiercely for him to remain, comparing him to Oak, who also needs extra help but whom the family would never send away. She invokes the concept MaxiMum identifies as neurodiversity, the idea that minds work differently and that difference is acceptable. "He's our plus one," she insists. "Like it or lump it."
Grumps stays. He begins emerging from his room, playing piano, joining Wood on walks, and establishing a daily marble routine with Brian. He builds Brian a cardboard Spitfire airplane when her fire truck is lost. He discusses old books with Sumac and accepts a GPS tracking watch without fuss. PopCorn transforms the attic into an ice palace with silver walls and fairy lights, and Sumac accepts the space as her own. At the Rakhi celebration, a Hindu festival honoring the bond between siblings, the family ties threads on each other's wrists and includes Grumps. Brian insists on both receiving and tying threads, declaring she is "sister and brother." Grumps, chatting with Gram (MaxiMum's mother) and PapaDum's parents, recalls being evacuated from Glasgow during the Blitz, the German bombing campaign against Britain in World War II, as a nine-year-old. Sumac reflects that displacement without consent has happened to him twice and wonders whether dementia is the Blitz and the Lotterys are the island he has been evacuated to. When Catalpa orders 11 slices of watermelon at an outdoor movie, Sumac corrects her: "Actually, sorry, twelve," publicly counting Grumps as family.