The task force comes under intense scrutiny when the FBI suspects that an insider is selling covert information, so a reluctant Bailey agrees to a mock field training exercise designed to root out the mole -- but he learns too late that one of the agents (guest star Michael Gaston) has a personal grudge against him. When one senior investigator (guest star Jack Conley) is found murdered in the command center, Sam tries to profile which one of the team's edgy members might benefit the most from trading FBI secrets. Meanwhile, Grace goes public with some personal news, and Nathan stands at the crossroads of the future of his bumpy marriage.