One day, a mysterious woman in the street grabs Monk's attention. Monk has never met the woman before, but she seems strangely familiar, and he feels compelled to chase her for two blocks before losing track of her.
The encounter so haunts Monk that he can't sleep for days. Growing desperate, Monk takes Natalie's advice and decides to go for a late-night walk, during which he sees the mystery woman for a second time. This time, the woman is driving a taxicab. Monk tries to stop her, but she eludes him once again.
Then, Monk hears angry voices coming from an alley. He follows the noise to a restaurant window where he sees an apparent drug bust going down. An undercover cop is pointing a gun at two men: a longhaired drug dealer and a bookish man. A fight ensues, and the undercover cop is shot dead by the drug dealer. The bookish man and drug dealer run away.
Monk rushes to a pay phone and reports the crime, but by the time Stottlemeyer and the police arrive on the scene, there is no body, no blood, and no sign of a struggle. Stottlemeyer thinks that Monk, in his sleep-deprived state, is seeing things, and suggests that Monk go to a bar, have a drink, and get some sleep.
Monk listens to Stottlemeyer and heads to a bar, where he meets a con man named Gully. Monk tells Gully about the mysterious taxi driver, and Gully thinks he can help. He tells Monk to go to a train station and talk to a taxi dispatcher named Essie, who might know the woman Monk is looking for.
Monk finds Essie, who explains that the mystery woman's name is Maria Cordova, and that she'll be arriving for work at 4:30 in the morning. Monk decides to wait in the train station, where he's amazed to see the undercover cop from the drug deal! The cop is alive and well, and in the process of throwing something away.
Again, Monk calls the police, and again, Monk appears to be losing his mind when the supposed undercover cop turns out to be a man named William Lee. Lee is not a cop, he hasn't been shot, and he claims to have never been to the restaurant where Monk saw him. But Monk insists that Lee is the guy he saw earlier. Monk roots through the trash, where he finds some plastic coin envelopes from a coin shop belonging to a man named Jacob Posner. Monk remembers finding a money clip with the initials "J.P." at the restaurant, and Stottlemeyer reluctantly agrees to take Monk to the coin shop to investigate.
When they arrive at the coin shop, Monk recognizes the shop's owner, Jacob Posner, as the other, bookish man who was present at the drug deal. Stottlemeyer explains the shooting Monk witnessed, but Posner claims to have been in bed all night, and Stottlemeyer believes Posner.
Frustrated and confused, Monk heads back to the train station in search of his mystery woman, Maria Cordova. Monk misses Maria, but he's just in time to find William Lee lying dead from a gunshot wound. When Monk hears that Lee was shot with a twenty-two-caliber pistol -- the same kind of pistol Jacob Posner keeps in his coin shop -- Monk finally knows what's been going on.
The whole drug deal was an elaborate con, with Posner as the mark. The "drug dealer" and "undercover cop" staged the drug deal and shooting to make Posner think he was in serious trouble. The "drug dealer" offered to keep Posner in the clear in exchange for a valuable collection of Posner's coins. It might have worked if Monk hadn't alerted Posner to the fact that the supposed undercover cop was still alive. Posner realized he'd been conned, caught up with Lee at the train station, and killed him for real.
With William Lee dead, Posner returns to the restaurant to settle the score with the "drug dealer." Monk arrives just in time to prevent any further violence, and the police soon arrive to take all the bad guys into custody. Stottlemeyer, unable to drive after having a few drinks, shows up in a taxi driven by Monk's mystery woman - Maria Cordova.
At last, Monk is face-to-face with Maria, and he finally realizes what's been haunting him. Trudy was an organ donor, and when she died, Maria received Trudy's corneas. Monk is overwhelmed with emotion. After being apart for years, Monk is able to look into Trudy's eyes once again.