Warning: This synopsis contains plot spoilers
London's National Gallery houses some of the world's finest masterpieces, and its curator, Quentin Lester, wants nothing more than to live among them, distraction-free, in a pure and simple life of the mind. When the Gallery's Victorian-era plumbing fails and floods the museum, its paintings are brought to safety in an abandoned slate mine in Manod, North Wales — the very mine to which the collection was evacuated to during World War II. Quentin accompanies his beloved Raphaels, Titians, and Velasquez to safety, relishing a chance to tend to them in isolation.
That isolation, in the grey mist and dramatic slopes of Manod, brings with it sheep; a vaguely frightening butcher; a charming and spirited, if slightly nosey, local schoolteacher, Angharad; and a 10-year-old boy, Dylan Hughes, with whom Quentin develops an unlikely friendship. For Dylan, whose father has just left the family in the face of devastating financial woes, the privileged outsider and his convoy of trucks from London represent a chance to save the Snowdonia Oasis Auto Marvel garage, his family's business. For Quentin, who mistakes the boy for an art connoisseur when a chicken-wrangling incident goes awry, Dylan presents an opportunity for the human connection that this urbane aesthete craves.
The masterpieces, stored in yellow crates — to the consternation of the ever-challenging teacher Angharad — inspire not intellectual contemplation but action of all sorts throughout the sleepy village. When, out of desperation, Dylan and Minnie, his aspiring criminal mastermind little sister, perpetrate the art heist of the century, it's the renowned curator who gets a lesson in art appreciation and the power of art to transform lives.