Gail Segalis and her new boyfriend have just returned home from a trip to Europe. To celebrate their one week anniversary, Gail uncorks an expensive bottle of wine. To her surprise, there's something inside - but it's not wine. Her boyfriend, who just stepped out of the shower, flies into a rage and strangles her with a towel! Gail Segalis falls to the ground, dead.
The next day, Monk - getting his grocery shopping done - bumps into a fellow shopper. They hit it off immediately. His name is Hal Tucker. Hal is everything Monk ever wanted in a friend, and the two men promise to meet again soon.
Back at Gail Segalis' house, a full crime scene investigation is underway. Monk is there, and it turns out that he knew the victim. Gail was a former police dispatcher who used to car pool with Monk. She would tell him about all the trips she wanted to take, and promised to send him a picture if she ever got to the Parthenon. Then, in the middle of the investigation, Natalie receives a call for Monk. It's his new friend, Hal! Everyone is stunned to hear that Monk has a friend, but Monk couldn't be happier.
The following day at Monk's apartment, Hal arrives to pick up Monk on their way to a hockey game. Natalie and Julie are there, and they find Hal to be charming and friendly and normal - the exact opposite of Monk! Naturally, Natalie's suspicious. Monk and Hal get along at the hockey game like a couple of old pals. However, Hal runs into another friend - an ex-con who did time with Hal. Monk has his first doubts, but quickly forgets them - after all, he has a friend!
Stottlemeyer decides to interrogate Gail Segalis' former boyfriend, Tim Hayden. Hayden explains that Gail left him in Greece for another man - one who bought her an expensive bottle of wine. Back at Monk's apartment, Monk and Hal watch an old kung fu movie - interrupted only by the arrival of Monk's mail, which Hal eagerly picks up.
Later that day, we see Tim Hayden again. He's alone, working in his gourmet restaurant on an off day. Suddenly, a mysterious figure barges in and knocks Tim backwards. The intruder is none other than Monk's new friend, Hal Tucker! Far from the cheery good guy we've come to know, Hal is an icy killer! He viciously clubs Hayden to death with his gun, and calmly leaves.
After a session with Dr. Kroger, Monk decides to take his relationship with Hal to the next level - he's going to ask him to be his best friend. Hal tags along when Monk gets called to the murder scene at Tim Hayden's restaurant. Hal notices a pair of sunglasses he dropped earlier and surreptitiously retrieves them. Stottlemeyer, meanwhile, has also been harboring suspicions about Monk's new friend, and he, Natalie, and Disher take Hal aside and confront him. Hal defends himself, claiming to be Monk's only true friend.
Back at Monk's apartment, Hal once again checks the mail, and this time, he finds what he's been looking for. It's a letter from Gail Segalis which contains the promised photo of her at the Parthenon - and Hal is in it! If anybody saw that photo, they'd know that Hal was Gail's new boyfriend, and her likely killer. Hal pockets the incriminating letter. Suddenly, Hal has no need for Monk, and when Monk asks him to be best friends, Hal hastily agrees before hurrying off. Monk is ecstatic. He thinks he has a new best friend!
But Monk becomes a wreck as days pass without hearing from Hal. Natalie convinces him to go to Hal's place and talk to him, man to man. Meanwhile, Stottlemeyer makes a connection. While at the crime scene, Hal was told that Tim Hayden was killed by a gun. But Hal knew that Hayden was clubbed to death, not shot. There was no way Hal could have known that... unless he was the killer! Meanwhile, Monk arrives at Hal's and finds the bottle of wine that Gail Segalis wanted to open before Hal killed her. The bottle contains an antique figurine, and Monk starts thinking. Hal must have been smuggling antiquities into the United States, and when Gail found out, she had to die. But just as Monk is putting this all together, Hal turns on the charm and becomes Monk's best friend again. He wins Monk over, then pulls a gun! At the last moment, Stottlemeyer and Disher bust in to save the day.
Hal is arrested, and it's another case solved for Monk, but this time, he loses a best friend in the process. However, Monk comes to realize that he's been surrounded by real friends all along: Natalie, Captain Stottlemeyer, and Liutenant Disher.