Michael's family has just moved to an old fixer-upper. But his baby sister is in the hospital with a heart problem, and Michael feels devastated and helpless.
When he sneaks into the crumbling garage, Michael finds a stranger named Skellig living (or apparently dying) there, a man immobilized by arthritis, subsisting on insects and spiders, and surrounded by owl pellets. While helping him, Michael discovers that the man is oddly light and has strange growths on his back that may be wings.
As Skellig begins to inhabit Michael's dreams, he and his new friend, Mina, help Skellig into an abandoned house. There Skellig seems to have an odd relationship with the owls, who bring him food. And as Michael's mother keeps vigil by the baby's hospital bed, Michael begins to feel his sister's heart beating within his own, and Skellig appears in his mother's dreams as well.