There’s no actress on television to compare with captivating and luminous Mary-Louise Parker, who single-handedly makes Showtime’s “Weeds” an impossible show to miss. She defines the series the way James Gandolfini has come to personify the spirit of “The Sopranos,” and exerts an almost magical, mystical hold on us that compels us to keep watching.

It’s so rare to see a woman on television who isn’t controlled or defined by men, and Ms. Parker’s acting triumph, coupled with smart writing and sophisticated storytelling from the talented “Weeds” production team, sets a new standard for the rest of television, including HBO, to uphold.

As it begins its second season this Monday night at 10 p.m., “Weeds” transforms from a quirky, fascinating experiment into a fullblown television hit, likely to last for as many seasons as Showtime can milk from its cast and crew. The stakes have been raised, and just in time; as last season ended, it remained unclear whether the show’s smalltown, small-potatoes concerns would remain that way at the risk of boring audiences in search of bigger, richer themes.

Season 2 begins only moments after it Season 1 left off, but suddenly matters have turned serious. Marriage and politics and crime have now trickled down onto the little boxes of Agrestic, Calif., and no one will ever be the same. Without giving away the multiple surprises in store for loyal “Weeds” viewers, suffice it to say that ample twists and crazy turns await (and for those arriving late, it should be noted that the “Weeds” editors deliver the best “scenes-from-last-episode” recaps on television, bar none).

The introduction of Martin Donovan as Peter Scottson, the new love interest for Ms. Parker’s pot-dealing Nancy Botwin, recharges the show in ways that can’t be calculated in a recitation of events. Over the course of the five episodes made available for review, his character — a DEA agent — involves her in more ups-and- -downs than most marriages endure in a decade. And that’s the point; Nancy’s life is a series of highs and lows, literal and figurative, as she explores ways to expand her drug business, satisfy her romantic yearnings, and be a loving and involved mother — and all within the narrow confines of her pretty suburban world. Right from the start of the first episode, she’s in full command, and by the end of episode five, even as everything in her life has veered out of control, she still manages to drive stakes into the ground to keep her world from blowing away.

This season, it’s Nancy’s sons who tax her patience the most. Their adolescent behavior and demands force her to confront issues of puberty and sex that would be enough to devastate most single mothers. But her quiet confidence staves off catastrophe, as does her crazy yet oddly grounded brother, Andy (played with great wit by Justin Kirk), who helps out when not pursuing a career as a Rabbi. Celia Hodes, Nancy’s friend, decides to run for City Council against Doug Wilson, and the battle between the comedy chops of Kevin Nealon and Elizabeth Perkins in those roles takes the pressure off Ms. Parker to deliver all the laughs.That liberates her to be what she deserves to be, and what we want: the warm, emotional center of a whacked-out world. Nancy’s friends and family, and we, depend on her perfect balance between sanity and silliness.How can you not love a woman who wears a T-shirt that says, “fear and loathing and fear and loathing and fear …” to a wedding ceremony?

The triumph of “Weeds” comes in its understanding of strong, independent women who know what they want. Credit for that goes to Jenji Kohan, the show’s gifted creator, who sensed the huge vacuum on television and filled it spectacularly. Even Edie Falco’s Carmela Soprano, one of the greatest female television characters in years, doesn’t approach Nancy Botwin in terms of self-awareness and insight.

It’s impossible to separate the role from the actress; knowing that Ms. Parker is herself a single mother — having been publicly abandoned by an idiot actor (Billy Crudup) and forced to face that cruel juncture in an actress’s career known as the 35th birthday — adds to our appreciation of her decision to take on this wonderful role.You won’t find a performance to top hers anywhere on television, or a role better suited to this moment in history, all of which transforms “Weeds”from a show worth catching into one worth remembering.

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Meanwhile, on HBO, it doesn’t get much worse than last Sunday night’s episode of “Entourage.” A season that showed so much promise turns out to have less bite than my pet Chinatown turtles.

Taking the wrong page from the “Sex and the City” playbook, the producers of “Entourage” have turned the spotlight on its subsidiary characters, most notably (and most regrettably) “E” — Vince Chase’s manager — who found himself locked in combat with Seth Green over issues of penis size and his girlfriend’s affections. Johnny Drama’s odd encounter with a male massage therapist only added to my sense that the show has wandered way, way off course.What happened to the story arc concerning Vince’s career? The “Entourage” team seems to have forgotten that Vince is the reason we even care about these guys. The show’s generic writing and lazy storytelling have made “Entourage” the biggest disappointment of the summer.

dblum@nysun.com