In July 2020, seventy-two-year-old retired adman Vic Trenton, recently widowed, is staying at his old friend Greg Ackerman's McMansion on Rattlesnake Key, Florida, to grieve his wife Donna's recent death from cancer. He meets neighbor Alita 'Allie' Bell, an elderly woman who pushes a double stroller containing only shorts and shirts representing her twin sons Jacob and Joseph, who died from rattlesnake bites in the early 1980s. Vic learns from Greg and later caretaker Mr. Ito how the boys wandered into a snakepit, prompting a massive 'snake hunt' that eradicated the rattlers by burning them at the Key's northern point. Allie brings Vic cookies, and he indulges her belief in her invisible children. Decades earlier, Vic and Donna had lost their own four-year-old son Tad to dehydration in a hot car besieged by a rabid St. Bernard; they divorced, then reunited and remarried before her cancer death.

One morning Vic finds Allie dead by her mailbox, being torn at by buzzards. The autopsy confirms a heart attack. A handwritten will leaves her entire estate—worth millions—to Vic, witnessed by her landscaper. Deputy Andy Pelley (Super Gramp) suspects Vic of foul play and orders him to stay on the Key. The stroller mysteriously reappears at Greg's house repeatedly, with different shorts and shirts on it. Vic begins experiencing hauntings: squeaky wheels in the night, shadows, a vision of the bathtub filled with rattlesnakes, and finally seeing the twins as grotesque adult bodies with bloated, snakebitten children's heads in the guest bed. Their thoughts—'see us, roll us, dress us'—invade his mind, demanding he take Allie's place as their caretaker.

Vic realizes the twins, not Allie, engineered his entrapment, choosing him as their next surrogate parent. He attempts to summon his dead son Tad at the gazebo and finds the snake pole moved with 'PR' scratched into the floor—a clue pointing to the pram as the link binding the twins. Dressed against snakebite, Vic pretends to take the twins for a walk and pushes the stroller to the Key's northern shell beach, fighting off real and phantom snakes and visions of fire. He flings the stroller into the whirlpool of Daylight Pass, where it sinks and the twins' shrieking presence vanishes. Across the pass, he glimpses Duma Key restored, with a tall man—seemingly grown-up Tad—waving to him.

The toxicology clears Vic, the inquest closes, and after probate he inherits Allie's estate, donating most to a food bank. Writing in 2023, he reveals the county is planning to reclaim Duma Key by closing Daylight Pass, which will eventually unearth the sunken stroller. Vic prays the twins are truly gone—and that they have no interest in him if they are not.