A withdrawn teenage boy, preferring solitude and private fantasies, lies to his parents about going away on a ski trip and hides in the cellar of his apartment building; his plan unravels when his estranged half-sister unexpectedly joins him, and over the course of an intense, intimate day they strip away pretenses, confront family fractures and personal fears, exchange confessions and small acts of tenderness, and forge an uneasy but transformative bond.