Marissa Irvine arrives to collect her young son Milo from his first playdate, but the woman who answers the door isn't a mother she recognizes 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.

Alcaras struggles to balance work and home life and must make a difficult choice when he's tempted by an offer that goes against his morals. The police struggle to connect the mystery marathon man with Carrie and Milo.