Pete Riley stays at home; his partner, Maddie, is the breadwinner. He spends his days browsing parenting blogs, where no concern is too trivial, and pacifying their rambunctious son, Theo.
Then, one day, a knock at the door. Miles and Lucy, a posh and near-perfect couple, tell Pete something shocking: Theo isn't his son. Their children were switched at the hospital.
At first, the couples are determined to reach a mutual agreement. They're all nice, rational people--surely they can sort this out between them. But soon their precarious arrangement--of babysitting, play dates and shared parenthood--begins to erode under the weight of perceived slights, hidden anxieties and petty jealousies.
It isn't long before Miles reveals himself to be cold, commanding and aggressive. When he brings a custody case against the Rileys, suddenly their parenting abilities are under suspicion and their private lives become ammunition. That's when their damaging secrets are exposed, their relationship tested to its breaking point.
They might teach their son to share and behave, to say "please" and "thank you," but when it comes to protecting their little family, Pete and Maddie are through with playing nice.