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If you have a good internet connection, you can watch the whole episode at nbc.com by tomorrow probably. Otherwise, here's the summary.

A young couple comes to the city to see the Halloween parade, and while the mother is in a bodega buying diapers she leaves her baby in the car. When she comes out, her car has been stolen and the baby is gone. Amaro had been taking his daughter trick-or-treating and Olivia had been spending time with her former foster son Calvin, but they're both called in to work the case. At the scene, the dad flips out when he hears that the mom left the baby alone, so the detectives separate the two of them and assume the dad did it.

A tip line is set up, but hours go by with no viable leads. They keep the parents in separate rooms to see if their stories match up, and because they find out that the dad had a domestic violence complaint up in Buffalo. Cragen gets suspicious that no one except the parents ever saw their baby with them while they were in the city, so Fin and Rollins go upstate to try and figure out when the baby was last seen alive. Meanwhile, Benson and Amaro get a tip that the couple's car has been found abandoned with no sign of the baby.

A man comes in with the baby's car seat claiming that he bought it from a peddler on the street. Benson and Amaro find the peddler, then determine that he broke into the abandoned car and stole all the stuff in it to sell. Among the items he took was a cooler, in which the lab found traces of fecal matter with the special kind of formula that the baby drank. At that point they know the parents did something to the boy, they just need to get them to admit it.

They still keep the parents in separate rooms so they can't rehearse their stories, and both parents admit to killing the baby, each saying that he died in a different way. The mom draws them a map of where they buried him, and they finally find the body buried in sand on a beach. Melinda Warner examines him and thinks he died of SIDS, Sudden Infant Death Syndrome, so the parents wouldn't be culpable. But she isn't sure, so Amaro suggests questioning them further to find out what really happened.

The mom admits that she took the baby to bed and put him to sleep between her and the dad. She was careful to put him on his back and not have any blankets or pillows around for him to accidentally suffocate on, but he died in his sleep anyway. When she woke up and he wasn't breathing, she panicked and thought she'd rolled over in her sleep and crushed him or something. She couldn't revive him, so she and the father came up with the plan to report him missing in a carjacking because she didn't think anyone would believe that she hadn't meant to kill him. Amaro finally explains to her that he'd died of SIDS and it wasn't anyone's fault, and she and the father are allowed to see each other. Fin and Rollins still want them charged with something for wasting the city's time on a false report, but Benson and Amaro argue against it, because they've already suffered enough.