The best new midseason series isn’t on the networks, or even on much-acclaimed HBO.

As hard as it may be to believe, the series is on FX, the cable channel whose only previous original entertainment programming was Howard Stern’s raunchy “Baywatch” send-up (and guilty pleasure) “Son of the Beach.”

The new show is “The Shield,” which begins a 13-week run at 10 tonight. This police drama is as cutting edge as “Hill Street Blues,” “NYPD Blue” and “Homicide” (which it most resembles) were in their heyday.

“The Shield” tells the story of the members of a precinct in a tough section of Los Angeles. Its young commander is Capt. David Aceveda (played by Benito Santiago), who has his sights set on a political career.

Acevada’s biggest obstacle is effective but corrupt cop Detective Vic Mackey, who heads up the squad’s elite Strike Team unit. Mackey regularly bends the law to accomplish his goals, but he’s well connected higher up in the department.

Two male-female pairings are featured as well. Claudette Wyms (CCH Pounder) is the veteran detective who knows how to close cases. Her partner, Holland “Dutch” Wagenbach (Jay Karnes), fancies himself as a bit of a psychological profiler. Officer Danielle “Danny” Sofer (Catherine Dent) is breaking in new officer Julien Lowe (Michael Jace).

What makes “The Shield” a cut above the usual police procedurals is that it makes Mackey a flawed protagonist, around whom the series turns. Chiklis, best known as the warm-and-fuzzy police commissioner on “The Commish,” is riveting as the mercurial Mackey, who operates under his own set of rules ultimately for the benefit of society. He’s a vigilante with a badge, which isn’t everybody’s cup of tea. But the setting of “The Shield” is a gray world, with no blacks and whites.

The drama takes full advantage of the looser standards on cable, with profanity, adult situations and occasional nudity, giving it a more realistic feel. “The Shield” makes “NYPD Blue” sound like “Adam-12.” Fans of light-hearted mysteries such as “Murder She Wrote” or “Diagnosis Murder” need not tune in.

“The Shield” is important because it pushes the envelope of what TV can be. It’s the kind of experiment that viewers could previously only appreciate on premium cable channels such as HBO. It’s also a challenging ride for adventurous watchers.