Even "Diary of a Mad Housewife" never had an entry like this one: the heroine has a husband, two children and four personalities.

In Showtime’s "United States of Tara," Toni Collette ("Muriel’s Wedding," "Little Miss Sunshine") plays a woman with dissociative identity disorder, which was once known as multiple personality disorder. The show’s comic conceit is that Tara’s loved ones treat her illness as an unenviable but livable condition - like diabetes - and humor her multiple personalities as old family friends or pesky neighbors. It’s not played entirely for laughs. And that is why Showtime’s new half-hour series labels itself a "dark comedy." A "light comedy" is a sitcom that finds its cultural collision by plopping an alien or a magical creature into the middle of suburban, middle-class America, like "Mork & Mindy" or "Bewitched." Premium cable networks have rejiggered the formula, taking instead a rare and alarming abnormality - polygamy on HBO’s "Big Love," serial killing on "Dexter" on Showtime - and mainstreaming that unthinkable way of life into PTA meetings, office picnics and suburban cul-de-sacs. These kinds of series mix the comedy of the protagonist’s battle to blend in with the quasi-tragic struggle to protect and preserve his or her true identity. Or, as is the case with Tara, identities.

Tara is depicted neither as a freak nor as a victim but as a valuable, lovable woman who happens to be burdened with more than her fair share of mood swings. When she is not T, a rowdy, sex-crazed teenage girl; or Buck, a beer-swilling, gun-loving redneck; or Alice, a cake-baking, ’50s-style homemaker, Tara has a career as an artist, painting decorative murals for rich women. Early on, the show establishes that Tara only recently stopped taking medication that suppressed her other identities because the drugs were deadening her artistic inspiration and her sex drive. By letting her repressed selves free, she figures, she may also finally get to the root of the childhood trauma that caused her personality to split and multiply. Her understanding husband, Max (John Corbett), is such a trouper that he agrees not to have sex with any of what she calls her "alters," even after T offers him a "Lolita moment" on the couch. "Tara" was created and written by Diablo Cody, who wrote "Juno." Not surprisingly, Tara’s two children, 15-year-old Kate (Brie Larson) and 14-year-old Marshall (Keir Gilchrist), stand out instantly: they are Junoesque characters, funny and touching in very different ways. Both are urbane, smart-mouthed and kindhearted.

Kate is pretty, slightly but not consistently rebellious, and interested in clothes, boys and having fun. She accepts her mother’s condition, except when she resents it. "Why can’t she just be manic-depressive like all the other moms?" she laments. Marshall is more of a loner. He is well read, listens to Thelonious Monk, bakes cupcakes ("It’s Paula Deen’s recipe, tweaked slightly") and is gay, and not afraid of letting his family know it. Ms. Cody begins Tara’s story where most multiple-personality movies end: after a diagnosis has been made, and the patient has resumed a somewhat normal life. The film "The Three Faces of Eve" and the television movie "Sybil" focused on the dynamic between patient and psychiatrist and their joint struggle to solve the mystery. This is a dissociative identity disorder story for the post-Freudian era: Tara’s therapist is rarely seen, and seemingly not much of a factor in her life. She has a far more intense relationship with her insecure sister, Charmaine (Rosemarie DeWitt), who cannot accept her sister’s condition, and resents the attention it brings her.

Actresses love to play characters with multiple personalities. Joanne Woodward won an Oscar for "The Three Faces of Eve"; Sally Field won an Emmy for "Sybil"; and Cynthia Nixon won an Emmy for playing a woman with multiple personalities in a guest appearance on "Law & Order: Special Victims Unit." Unfortunately for Ms. Collette, the roles of Tara’s children are so deftly written and skillfully played that they undermine her own star turn - Tara has four personalities and is one-dimensional in all of them. Her alters are caricatures, and while grotesque exaggeration may all too often be the case in real life, a drama requires more subtlety. If Tara’s family can readily accept and at times even have fun with the "alters," so should viewers.

Ms. Collette’s depiction of T is particularly hard to watch because Ms. Larson plays a real teenager so well and naturally: T and her gum-chewing, thong-thwacking tics look out of place, a "Saturday Night Live" sketch grafted onto a Sheridan play. Alice is not much more convincing; Ms. Collette plays her as a parody of Bree, the obsessive homemaker in "Desperate Housewives." Oddly enough, Ms. Collette is most affecting as Buck, a blustering redneck who has a hidden soft side. The baseline Tara, understandably exhausted by the "Night at the Opera" cabin scene crowding her brain, is wan and not terribly compelling. "United States of Tara" has a big premise and some wonderful small touches, but strangely, Tara is the least of them.