Wow. I'd have to say this is one of my favorites. It's certainly the best of the season to date, in a season that's been improving by leaps and bounds. So if a few mediocre episodes guarantee we get stuff like this afterward, lets have more "Takes Manhattan" and "Panic Rooms".
This was very much a Tony Shalhoub episode and honestly, I'd suggest the producers submit it as the viewing tape for his Emmy nomination. The writers have a tendency to fill in with "Adrian" scenes, and often use overacting or OCD-heavy bits. But newcomer Ross Abrash deftly makes Adrian...well, normal. Or almost normal. For one thing we get to see him function despite his handicap, instead of just blindly panicking or collapsing or whatever. Monk has to go undercover in a everyday working environment, and that's exactly what he does. His handicap doesn't really slow him down - in short, he can function. He has quirks, like the rest of us, but he isn't shown here as a basketcase. He seems able to do that in "Goes to Jail," and we had a sympathetic back-up character of dubious background there, too.
I suppose you could argue that Monk isn't very consistent here, given that he lapsed into a near coma in a confined space before ("Panic Room"). But myself, I'd rather ignore a bad story for establishing character motifs then ignore a good one.
The episode wouldn't have worked without the back-up character of this episode, as played by Enrico Colantoni. Best known from Just Shoot Me, since leaving that show he's branched out with guest appearances in Stargate and Justice League, honing his craft. It pays off here as he develops a deep and well-rounded character. We first see him as a shy and/or nervous man holding back while the police take over, and then moving into his own as the investigation gets underway. The idea of meeting Monk's partner is a good one (which, fortunately, they've avoided doing to date so we could have it here).
The camaraderie Colantoni and Shalhoub presumably developed while working together on Galaxy Quest also pays off here - both of them expertly sell the fact they were partners, and we get another glimpse of Monk's pre-Trudy's death career. We get both how competent he was as a detective and what he was like personality-wise ("He hasn't laughed since"). And again, how he could function as a policeman and not drive a partner crazy. The handshake is the big payoff, but the scene afterward where they're pondering the Employee of the Month rewards is both hilarious and a sign of them being able to work smoothly together. Hopefully Colantoni can spare time from his new show this far (Veronica Mars) to show up on Monk again - there wouldn't seem to be any reason why he couldn't appear as an investigating officer.
The regular supporting cast is there, and the Disher subplot is amusing enough - granted the ending isn't that surprising. But Abrash doesn't write him as a buffoon and has some sympathy for the character. Even if going to Arruba for a wallet-photo shoot sounds a little odd...
Bitsy Fleming doesn't have a lot to do but she has a nice rapport with Colantoni in the cafe scene. Ted Levine is average, albeit average by his standards. He's strong and supportive both in the bubble-wrap scene with Shalhoub and in the apology scene with Colantoni.
The rest of the guest cast is competent enough, particularly Patrick Thomas O'Brien as the seemingly-sincere store manager selling the virtues of the Employee of the Month awards with a straight face.
The Mystery? This is a more of a "What's the motive?" then anything - there's no big secret about who the murderer is. It's clever enough, and all the pieces are there, but this is definitely one where the mystery is less important then the subplots.
Actually, the secondary mystery (who stole the drugs?) was more interesting and it's another bit of the show that entertains and impresses and strengthens the Monk character - as in "Paperboy", time and space have no bounds on Monk :) . He solves mysteries in other countries by reading a newspaper, and here he solves a 30-day year old mystery by looking at stamps, and a 4-year old mystery just by looking at a photo and an auction report.
Not a whole lot of catchy quotes, but the ones that were, were good.
Downsides? Not a whole lot. The dog scene (reminscent of the '73 TV movie Trapped with James Brolin) and to a lesser degree the gun scene (a play on the ammo-buying scene in Bowling for Columbine, maybe? - the show is filmed in Canada) seemed a tad unnecessary and extraneous padding more then anything.
Overall, I'd rate this one of the highest of the show - a near perfect 10.