"Sick Day" opens when JJ arrives home after an exhausting case. She gets in a fight with her husband over not calling her about their son being sick. He correctly surmised that her case involved children. She keeps flashing back to the events of the case. She tells her husband Will about the details, at his insistence that she get the trauma out of her system.
The team settled on a pattern of older sister and younger brother for the arsonist, and the girl's younger brother was kidnapped right after JJ and Luke left. They pinpoint an unsub, John David Bates, who has an older sister, Trisha Bates, that he had tried to kill with fire when he was younger and works in a fire-related profession. They determine that rejection by his sister was the trigger that set him off.
Watch it online at CBS.com