Monk is at Natalie's house when they're visited by Lt. Steven Albright of the U.S. Navy. Albright is one of Mitch Teeger's old pals, and he's come for Monk's help. One of Albright's friends, Jason Pierce, has just committed suicide, but Albright, suspecting murder, wants Monk to investigate. Monk agrees to help, but when he gets to the scene of the alleged crime, he's surprised to find a submarine.

Monk reluctantly gets on board to investigate.

Albright explains the circumstances of Jason Pierce's death. The sub's commander, Nathan Whitaker, called a drill and Pierce, the submarine's second in command, failed to report. Commander Whitaker and Lt. Albright went to Pierce's quarters, where they heard a gunshot from inside. Commander Whitaker barged through Pierce's locked door and found Pierce dead from a single gunshot wound to the head. Monk, eager to get off the cramped submarine, agrees with the Navy's ruling that Pierce had committed suicide.

Then, just as Monk notices a clue, an alarm is sounded. The submarine is submerging as part of a fleet-wide exercise. Monk makes a mad dash for the exit, but it's too late. The submarine is plunging to the depths of the ocean, and Monk and Natalie are trapped underwater.