Answer:13-year-old Conor O'Malley's mom (or "mum," as the Brits say), is dying of cancer, and Conor's having nightmares. In his recurring dream—SPOILER ALERT!—a terrifying monster tries to pull his mother down into a pit, and Conor's at the edge trying to hang onto her hands. Not that we know this at the beginning. This dream is, in fact, the thing we wait the whole book to find out.
Every time Conor has the dream, he wakes up at exactly seven minutes past midnight. So when he wakes up one night and there's an actual monster outside his bedroom window, you'd think he'd be terrified. However, it's not the monster Conor's been expecting, so he's kind of unimpressed.
This monster spends its days as a yew tree in the cemetery behind his house, but at night it turns into a terrifying tree-creature with skin made of leaves and slices of bark for teeth. But Conor's not scared, because the monster in his dreams is way scarier—at least until the yew tree monster demands the scariest thing of all: the truth about Conor's nightmare.
First, though, the monster tells Conor three stories on three separate nights. Though they seem a bit like fairy tales, the good guys turn out to be the bad guys, and vice versa. And there's a sinister twist: when the monster tells the stories, Conor acts them out in his own life, then realizes what he's done only after the monster disappears. One leads him to wreck his grandma's living room, and one causes him to beat a schoolmate so badly the kid goes to the hospital with broken bones and teeth. However, when Conor does something awful, nobody punishes him, because they're all walking on eggshells around the kid whose mom has cancer. It's easier just to pretend he doesn't exist.
Speaking of the cancer, the last-ditch chemo the doctors tried on Conor's mother is failing. It's a drug called Taxol (even though nobody ever names it), and it's made from yew trees. Conor thinks this must mean the monster has come to heal his mother—but of course, that's not the case at all. When it comes to Conor's mom, there's simply no hope.
Unfortunately, Conor's been in major denial about the fact that his mom is dying, and his parents encourage that denial by pretending everything's fine. They all keep pretending right up to the moment it becomes clear she only has a few hours left. That night, the monster steps in, because somebody's got to help this kid see the light.
Our favorite anthropomorphic yew tree changes the family's backyard into the world of Conor's nightmare. Just like in the dream, Conor sees his mom standing beside the cliff, and he sees the nightmare monster grab her by the feet and try to pull her down into the pit. Conor runs and grabs her hands, but the pit monster's too strong for him, and Conor lets her fall. He tries his best to hang on, but in truth, he just wants it to be over. And now he knows he's an awful person who needs to be punished.
But the monster's all, hold up, it's not that simple. Humans are complex creatures, capable of being both good and bad at the same time, just like in the stories. Finally, there's a moral: sometimes when caring for someone for so long wipes you out, it's okay to let go. In fact, you have to.
Then, in one of the most sob-inducing scenes in any book ever, the monster makes its branches into a nest so Conor can crash there. He wakes up just in time to go to the hospital and hold his mother's hand while she dies—which, though the book ends just before she actually passes away, we know happens at 12:07.
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