Will Freeman (Hugh Grant) is a carefree bachelor living in London, relying on royalties from his father's Christmas song to fund his lifestyle. He invents an imaginary son to meet single mothers, but soon forms a bond with Marcus Brewer (Nicholas Hoult), a socially awkward 12-year-old boy. As Will learns valuable lessons about life and responsibility from Marcus, they develop an unlikely friendship that helps Will grow up and find love with Rachel.