Explaining the year’s breakout TV hit.

In a season that’s seen reruns of nighttime dramas hit all-time ratings lows and seen dramas give way to ever more sensational reality-based programs, CBS’s CSI: Crime Scene Investigation has been an unusual success. The show’s growing popularity seems to springboard off the idiot-vérité lineup that has moved CBS from its third-place berth as the “family network” to its current Survivor-buoyed place at No. 1. CSI is the network’s standout drama and one of its few “scripted” (as the trades put it) shows to consistently crack the Nielsen top 20. And the show’s relationship to reality TV is more intimate than simple proximity.

Each week, CSI uses the forensic investigators of the Las Vegas Police Department to stage the televisual equivalent to word problems from a particularly gruesome standardized test: If a cigarette is placed next to a somewhat obese woman, how long will it take for her nylon nightgown to act as a wick and burn her down like so much candle wax? (A few hours.) Or: Three pipe bombs, one made of an inflow pipe, one made of an exflow pipe, and one made of galvanized steel are exploded. Which throws its shrapnel the farthest? (The galvanized steel one.)

In making use of such morbid trivia in the show’s exposition, CSI stands in stark contrast to the superficially—and acronymically—similar ER. Though lauded for its realism, ER’s staccato bursts of medical jargon are rarely explained. For all we know, they could be making it up: “20 cc’s of dipsoarmoural! I’ve got a patient in hydrocotowhatis!” I’ve been watching ER for years now, and all I’m really sure of is that “stat” means “really fast.” Or I think it does. (Click here for an ER doctor’s take on ER and here for a helpful explanation of ER jargon.)

CSI’s surge forward in popularity stems from its neat fit into a niche that prime time rarely fills: It is frankly, conventionally instructional. CSI is a grown-up’s Encyclopedia Brown story, in which crimes are solved not by looking into a suspect’s mind but by examining his apartment for evidence of a particular kind of muscid fly that breeds only in urban areas. I’m not making these examples up, by the way. CSI’ s Web site has more detail than many textbooks on such forensically useful topics as the electrothermal atomizer, leuko-crystal violet, and petechial hemorrhaging.

CSI’s commitment to realism pretty much ends with its treatment of evidence, however. This being a Jerry Bruckheimer production, the cast includes some stunning, preternaturally structured women—the CSI Web site, for some reason, provides cast members’ heights and weights. The Emmy-nominated Marg Helgenberger’s Catherine Willows is 5 feet 6 inches and 104 pounds. Jorja Fox plays Sara Sidle, who is 5 feet 8 inches and 107 pounds. But what really stretches credulity isn’t Willows’ body fat ratio; it’s that before becoming a crime scene investigator, her character was an exotic dancer.

It’s instructive to compare CSI with another surprise serial hit of a few years ago, one that also investigated strange doings in exotic, fantastical locales. In replacing the insubstantial and even ethereal hypotheses of Scully and Mulder (conspiracy theories and spectral phenomenon) with crudely physical, concrete evidence (insect larvae and petechial hemorrhaging), CSI is the anti-X Files—a show that doesn’t merely suggest that “The Truth Is Out There,” but actually tells us what it is.