Eleven-year old twin sisters Sage and Riley are sent to their grandmother’s wild permaculture home for a year while their mother works on a remote archaeological dig in the Yucatan, researching ancient Maya forest gardens. Expecting a boring year with grandma, the twins are instead initiated into a fragile, hidden universe beneath the autumn leaves; a rare colony of fireflies that bioluminesce for two years in the dark soil, dependent on a protective blanket of leaf liter to survive.

Across the fence, nine-year-old Barnaby Sterling is confined to a pristine, acid-green yard by his overprotective, nature-phobic father. Mr. Sterling copes with his wife's recent death by obsessively guarding a River Birch tree she planted when Barnaby was born. Barnaby navigates his grief by immersing himself in an augmented reality of digital butterflies, while real ones swirl above his head. The three children initially bond over autumn Paw Paw fruit, but their friendship deepens over a common longing for their mothers. Letters and video calls from the twins' mother connect their backyard discoveries to a broader hemispheric ecosystem and history, bridged by migratory hummingbirds and sapsuckers that fly from the Yucatan right into their grandmother's food forest.

As the solar wheel turns from the autumn equinox toward the winter solstice, environmental and emotional crises threaten grandma's vibrant sanctuary. A sudden hurricane in the Yucatan cuts off the twins' connection to their mother, while an army of leaf blower's marches forward to violently strep away the autumn leaf litter the firefly larvae need to survive the coming winter frost. As late autumn moves closer to the winter solstice, the children spy strange lights in the adjacent state park. They discover a UV flashlight among a scarred patch of native goldenseal roots; a sign of poachers plundering the fluorescent plants for the black market. By the vernal equinox, the children's efforts to record the thieves go terrible wrong, leaving Barnaby confined to his room and the children's fragile bond severed. With Barnaby confined once again, if the twins cannot heal the rift between the families before the summer solstice, an irreplaceable colony of light will be permanently snuffed out, and a grieving family will remain frozen in their sorrows.