The episode is titled "The Detail", even though the whole show, in every season, is about a detail with the characters we know. But this episode sets up that distinctive team of characters which we will grow to like and know for all the years to come. Most of the things that happen in later episodes and seasons begin here. While the first episode introduced us the characters themselves, the second one tries to bring them slowly together and to set up the structure and mechanics of the detail.
Of course, at first everything looks really bad. The office is a hidden, dirty storeroom, there's no equipment, the team's additionally hired extra personnel consists of failures and rarely anyone seems to be motivated to actually solve the case. As we learn again here, McNulty functions not only as the central character, but most importantly as the engine that keeps everyone else going, even it is by pissing everyone off (leading to the first time he utters his most symptomatic key phrase: "What the f**k did I do?"). We also see the origin of the bond between McNulty and Kima, a bond which becomes so essential later in the story, mostly because they recognize each other as being "good police."
We also get an important perspective on D'Angelo who is everything but a simple-minded gangster, because he has a very strong conscience and a sense of morals, even though he tries to repress them as good as he can. At the same time we realize how persuasive and charismatic his uncle is, without ever falling back on stupid clicheés of the nasty gangster boss who only plays nice but is really cruel and sadistic. Avon isn't like that at all. He is simple-minded to a certain extent, but he isn't stupid. He is an idealist (which, as we will see later, separates him from Stringer).
There is also the introduction of Pryzblewski, a character which, especially after this episode, we immediately characterize as the stupid hothead, who has no idea what he is doing. The pointless violence he lets himself down to, gives not even a hint to what kind of a man he will become over the years, which is perfect, because that transformation is most astonishing for himself.
We also get more glimpses of raging Rawls, the flawed relationship of Herc and Carv, a very very short glimpse of Freamon and a surprising look on the marriage of Cedric and Marla Daniels. What we get here is a couple that actually talks. It is not about gender roles or stereotypes, but simply about two people who can talk with each other, who are equals on a communicate level. This may sound strange, but it reflects how rare such a relationship is in modern fiction where partners are most of the time used as additional characteristics of one character instead of being an individual character themselves.
It must be odd to read about all the things that will happen later on, but "The Detail" does set up so many things, that it's hard not to talk about them, when you know the whole story. The point is, that, even on the first viewing, you can feel that setup. This is the episode that will really get you hooked, because it promises so much. And if one thing is for sure, than that the show certainly delivers.