When a 5 year-old Ryan Downing dies from exposure to a rare bacteria, VRSA, the police don't know if they're dealing with a one off incident or a planned attack. They first focus on the on the Downing's nanny, but she is quickly cleared. The medical examiner thinks the boy was likely injected with VRSA - she finds a small puncture mark - and when a junkie named Clarence dies of the same disease, his needles indicate they were used on the Downing boy. The boy's father Aaron was having an affair with a co-worker Theresa Copeland and becomes their prime suspect. In order to get the evidence to convict however, ADA McCoy has to lean on the company that likely sold him the bacteria in the first place.
—garykmcd