Will Lightman (Hugh Grant) is a rich, hip, irresponsible Londoner who, in search of available women, invents an imaginary son and starts attending single-parent meetings -- confident in both parties' inability to make a commitment. But when Will meets Marcus (Nicholas Hoult), the troubled 12-year-old son of a depressed single mother (Toni Collette), a quirky and unexpected friendship blooms as both Will and Marcus help each other grow up.