Pete Riley answers the door one morning to a parent's worst nightmare. On his doorstep is Miles Lambert, who breaks the devastating news that Pete's two-year-old, Theo, isn't Pete's real son - their babies got mixed up at birth.

The two families - Pete, his partner Maddie, and Miles and his wife Lucy - agree that, rather than swap the boys back, they'll try to find a more flexible way to share their children's lives. But a plan to sue the hospital triggers an investigation that unearths disturbing questions about just what happened the day the babies were switched.

And when Theo is thrown out of nursery for hitting other children, Maddie and Pete have to ask themselves: how far do they want this arrangement to go? What secrets lie hidden behind the Lamberts' smart front door? How much can they trust the real parents of their child - or even each other?