Monk, Sharona, Captain Stottlemeyer, and Lieutenant Disher arrive in New York City to pursue a lead in the murder case of Monk's wife, Trudy. Dale the Whale has given them the name of a man that was involved: Warrick Tennyson. But before they can even check into their hotel rooms, Monk becomes a witness in the shooting death of the Latvian Ambassador to the United States. Unfortunately, Monk only gets a glimpse of the killer's left ear as he flees the scene of the crime.
Initially, the lead detective on the case, the NYPD's Captain Cage, thinks the ambassador's killing was a political assassination. But a series of clues leads Monk to suspect something more. Meanwhile, the NYPD is refusing to help Monk talk to Warrick Tennyson, who is in protective custody for a separate case. Captain Stottlemeyer helps Monk strike a deal: if Monk helps the NYPD with the Ambassador's murder, they'll let him talk to Warrick Tennyson.
Monk meets with a suspect, a Latvian expatriate with a political grudge against the Ambassador. He turns out to be innocent - his ear does not match the killer's - but a friend of his provides Monk with a clue: the Ambassador's last words, spoken in Latvian just before he was shot, translates to "This is not my coat."
Unfortunately, before Monk can do anything with his latest clue, he gets lost on the subway and finds himself alone in Times Square, nearly losing his mind amidst the chaos of New York life. But Monk catches a break: while in Times Square, he sees a man, Steven Leight, being interviewed on a jumbo television screen about the murder of his wife. Monk recognizes Leight's ear - he's the killer!
It's only a matter of time before Monk and the police track down Leight. Monk puts everything together: Leight, after murdering his wife in Central Park, stopped into a bar for a drink, where his coat got mixed up with the Latvian Ambassador's. When Leight caught up with the Ambassador in the hotel elevator, the Ambassador realized he had the wrong coat, and noticed some of Leight's dead wife's jewelry in the coat's pocket. Leight shot the Ambassador and his bodyguards, and switched coats to prevent anyone from finding the incriminating evidence.
With the Ambassador's murder case now solved, Monk is allowed to meet with Warrick Tennyson, who turns out to be gravely ill. With his last, dying words, Tennyson gives Monk some new information about Trudy's murder: Tennyson had built the bomb that killed Trudy, and was paid to do it by a six-fingered man named Frank. Now Monk and the gang can return to the comforts of San Francisco with another case solved - and another step forward in the one case that haunts Monk the most.