Eve is leaving the SFPD, so Nash, Joe, and Harvey go out on a Friday night to celebrate. Harvey's car is vandalized and stripped, so they all must take the BART (Bay Area Rapid Transit) train back to San Francisco. Also boarding the train is a group of crazed robbers intent upon robbing a postal shipment of social security checks. The four robbers, headed by a young unstable girl, Carrie, are frustrated when the shipment was delivered the previous day. She decides to rob the passengers instead. Nash and company intercede, however Eve gets taken hostage in a standoff; the cops are outgunned and all get taken hostage. A passenger attempts to escape and is shot dead by Carrie. The train is brought to a standstill on the tracks.
Meanwhile Cassidy, Nash's daughter, has a severe stomach ache which appears to be appendicitis. Lisa, Nash's first ex-wife, takes her daughter to the hospital emergency room where they encounter a nightmarish situation in the waiting room.
Lt. Shimamura and Evan are brought in by the transit police. Nash is forced to be the negotiator. Carrie's demands are 2.75 million dollars or she will begin killing hostages in one hour. In a struggle with Carrie, Eve gets shot in the leg. Another passenger manages to successfully escape, however, one of the robbers gets shot by the police. Carrie orders Nash to get her wounded comrade medical attention - Nash uses the opportunity to get Eve out also. Carrie orders the train to move to another location. Against procedure, Lt. Shimamura sends Evan out to the train. Evan hooks up with Destin, a transit worker with a fast motorcycle to catch up with the moving train.
The ill Cassidy is treated with a pain killer which makes her high and giddy. Cassidy lies next to a crazed biker named Road Kill. The biker is restrained to the bed and begs Cassidy to release his straps. As Lisa watches, the wounded Eve is brought into the emergency room, and Lisa realizes that Nash is also in trouble.
Evan arrives at the train. Joe demands to take a much needed urination. While Joe takes a leak at the door to the train, Evan manages to hand Joe a gun. Evan rushes the train, and Nash overpowers one of the robbers and shoots the girl dead. Nash visits Eve in the hospital and discovers Cassidy there. Cassidy's pain was not appendicitis, but muscle spasms.
For more, see commentary for Episode 15