Jay Leno stops by the Waterfront Bar while in town visiting his cousin Mary; Munch and Bayliss don't want to bother him. A new year starts out with a clean board and Brodie begins his work taping with the detectives. A sniper takes the city hostage, threatening to and executing people every 8 hours. The sniper uses Hangman as a way to leave clues for the detectives. Bayliss is the primary and his back is giving him trouble as he worries about surgery his doctor wants him to have. To combat the pain, he keeps taking muscle relaxants. A suspect is found and cornered and he shoots himself before they can take him into custody. Russert gets demoted when Barnfather doesn't like the way she handled the investigation.

Mary gave birth to a girl named Olivia; Tim is more excited about it than Frank. Their latest case involves a multiple shooting a restaurant. Lewis is back from his honeymoon, but while working a case with Kellerman, he confesses that they have yet to consummate their marriage. Later he tells him that they've separated. Tim stops by to see Frank and Mary's baby. They get a suspect for their case in the box. Frank collapses in the box while interrogating a suspect. Frank is put under the knife to release the pressure built up in his head from the stroke he had. After the surgery the doctor tells them that he doesn't know if Frank will recover.

Night has fallen and the crisis is no closer to being resolved. Gee demands that Barnfather make a decision. Frank continues to trying to adjust. Frank insists on going to the hostage scene. The identity of the hostage-taker is made when he demands that his pig be brought to the school. When the pig doesn't arrive in time, he sets a fire and seriously wounds a hostage (who later dies) and then becomes badly burned. Brodie takes issue with Munch's treatment of Pembleton. The paperwork gets completed and the hostage-taker is formally charged. Frank makes a decision regarding his recovery.

Pembleton waits with anticipation for the phone to ring. It does and with Bayliss as the primary, they proceed to investigate the triple murder of a divorced woman and her children. Kellerman asks Brodie not to return to the boat. Lewis and Munch get a homicide that they can track back to Luther Mahoney. Bayliss and Pembleton disagree on who the primary suspect is in the murder. Mahoney's nephew wants to remain ""monogamous"" in implicating his uncle. Kellerman gets his summons to appear before the grand jury. Frank and Tim must work out the finer points of working in the box together again. Mahoney's hand reaches out from jail and silences his nephew; the case is a bust. Lewis lays down the law to Luther Mahoney in the Waterfront Bar. Cox finds Kellerman and his predicament intriguing and offers him some company, the night of his summons.

A woman that Munch had a crush on in high school is found murdered. He struggles with his faith as he works with Kellerman on the case. Meanwhile, Frank also struggles with his faith and the fact that Mary has left him.

There is a major police chase after an armed suspect, as he holes himself up in a row house that is the headquarters of ARM (African Revival Movement). The head of that house, a former cop, calls in a favor, high in the police department. Munch and Pembleton try to work the case as Bayliss seems to be missing. Their investigation seems to be thwarted every step of the way. Stivers' has second thoughts about what she saw at the Mahoney shooting. An attempt to serve ""Burundi"" Robinson an arrest warrant breaks out into a war between the police and ARM. Gee goes into the house to confront Robinson and finds out who and what he knows.

Al Giardello is shot and the detectives of the Baltimore Homicide Unit return to work to solve the case, including Howard, Munch, Bayliss, Meldrick, Bolander and Kellerman.