"The Shield," which returns Tuesday, has always been a riddle wrapped in a mystery inside an enigma, chained to a stack of dynamite, doused in kerosene, bordering the edge of a roaring campfire.

The ticking time bomb will finally explode in this seventh and final season with Vic Mackey, a detective whose moral code is as twisted and poisoned as an alcoholic's intestines, finally meeting his date with destiny.

It won't be a pretty ending. In fact, everything about "The Shield," including its grainy tone, twitchy camerawork (even the opening credits vibrate) and shady characters, has spit in the face of convention.

It's an approach long embraced by HBO, but back in 2001, when the FX network's experiment was being developed under the name "Rampart," nobody on basic cable was playing the rebel card. And as far as FX was concerned, it could have been a one-time bet.

"If 'The Shield' had failed, FX would have quickly exited the scripted-original-series business," said FX president John Landgraf. That would have meant no "Nip/Tuck," no "Rescue Me," no "Damages" -- and one could argue that it might also have meant no "Battlestar Galactica," "The Closer" or "Mad Men," all shows that premiered on competing cable channels after FX proved that the edgy drama could succeed.

When "The Shield" premiered in March 2002, there were eight hourlong scripted series on basic cable. This year, there will be more than 30.

The show's legacy will take a severe kidney punch, though, if the writers don't find a proper and memorable way to say goodbye to Mackey.

The series has boasted an unprecedented number of jaw-dropping moments -- the graphic rape of an ambitious politician, a harrowing journey inside a child-prostitution ring, the strangling of a stray cat by a "good guy" detective desperate to grasp the mind-set of a serial killer -- but none matches the shock at the end of the pilot episode, in which Mackey, hoping to protect his corrupt practices, turned around at the end of a police raid and put a bullet through the head of one of his squad members.

Mackey has performed a number of good deeds since, usually in the name of his family or streetwalkers, but none has come close to paying the price for that sin.

"He's a man plagued by regret and the fallout of his actions, starting pretty much from day one of the series," said Michael Chiklis, whose work as Mackey earned him an Emmy and allowed him to put his comparatively lightweight series, "The Commish," in the taillights. "He's definitely become a guy that understands there is a tremendous consequence, not just for himself, but for everyone around him, for the decisions that he's made. But you can't change the spots on a leopard. He's in the vortex and he's swimming."

Looking for the exit

Before deciding Mackey's fate, creator Shawn Ryan went back and watched all 75 previous hours of his series, a crash course that made him realize he still had some loose ends to tie up (die-hard fans will savor guest appearances by blasts from the pasts, although no one should hold their breath for visits from past cast members Glenn Close or Forest Whitaker).

Ryan admits that his original plans for the finale got scrapped while he was developing the fourth of the final 13 episodes.

"It's not an auteur system," Ryan said. "It's a very collaborative show. So we came up with a plan for this final season that we thought worked really well. Then we came up with a better idea, so we changed the plan. And that's always been the way that this show has worked best."

Fans who want to avoid any spoilers should turn immediately to the business section. For the rest of you, just know that the first eight episodes don't suggest that Mackey's arc will end on a rosy note. His daughter, played brilliantly by Chiklis' real-life offspring Autumn, has turned to drugs and kinky parties. A gang war he triggered has inspired murderers to slice body parts off their victims and strew them along the street. The tension with his onetime confidant Shane Vendrell leads to more than one assassination attempt.

As usual, Mackey sweats, schemes and slides out of one impossible predicament and into another, but it's clear that he's running out of exits.

"You have to pay some kind of price," says his separated wife, played by Cathy Cahlin Ryan.

No one involved in the show will spill the beans on exactly how it will all end, but CCH Pounder, who portrays Capt. Claudette Wyms, dropped the fattest hint:

"It blew my socks off," she said. "Vic Mackey gets what he deserves."

njustin@startribune.com • 612-673-7431