Marissa Irvine arrives to collect her young son Milo from his first play date, but the woman who answers the door isn't a mother she recognises and doesn't have Milo, beginning every parent's worst nightmare.

As Detective Alcaras investigates Milo's disappearance, Marissa, with Jenny's help, works hard to raise awareness; a press conference turns ugly when questions are asked about the Irvines' past business dealings.

Alcaras turns his attention to the recent Chicago Marathon, attended by Carrie, the Irvines and a potential new suspect; a canvassing event aims to keep the case alive; Peter takes matters into his own hands.

Detective Alcaras struggles with work-life balance and faces a moral choice; police try to connect the mystery marathon man with Carrie and Milo, but struggle to find a link.

News from Alcaras concerning Carrie's hideout throws suspicion on all of the Irvines; Brian uncovers a betrayal from Peter, who accuses Lia of lying; a long-buried family secret threatens to tear the Irvines apart.

The Irvines struggle to adjust to their new normal, while also recovering from the fallout of their arguments; Marissa tries to cope with past events; Jenny deals with domestic pressures of her own and uncovers some of Richie's secrets.