After nine years and 199 episodes of Law & Order: Special Victims Unit, Mariska Hargitay and Christopher Meloni were convinced they'd tangled with the most dastardly of criminals. Their latest guest star, however, turns scene-stealing into a case of grand larceny.

"Robin Williams," Meloni jokes, bristling with the intensity of his character, Det. Elliot Stabler, "He's really pushing my buttons, that f--ker!" In "Authority," the hit series' 200th episode (airing  April 29), Williams plays Merritt Rook, a slippery character, based on a real perp, who impersonates police officers. "He gets his rocks off commanding people to do very strange, lewd and lascivious acts," Meloni explains. "It's an interesting statement on blind obedience."

For Hargitay, who plays Det. Olivia Benson, the prospect of acting with Academy Award-winner Williams is exhilarating. "I think about some of the guest stars we've had, and it really is a milestone working with such a genius," she says, catching a breather from an interrogation scene with Meloni and Williams. "It makes me so proud that he wanted to do our show. Look how far we've come!"

It's been a long, successful ride for the frank and provocative drama, which is now the most popular vehicle in the Law & Order fleet and one of the most-watched syndicated programs on cable TV. "Our subject matter is painful and compelling," Hargitay says of SVU's continuing success. "It shows this flaw in humanity that keeps repeating itself over and over, but has to be looked at. And our characters' compassion and anger help people deal with their feelings about these horrible things." While other shows were hobbled by the writers' strike, SVU is returning stronger than ever. "I think we have the edgiest content on television," says costar Ice-T, one of the early architects of gangsta rap. "I've always been considered a hard-core person, and sometimes I'll read the script and go, 'Damn!'"

That's probably what Hargitay thought when she read the script for "Undercover," SVU's first post-strike episode. In it, Benson poses as a jailed junkie in order to take down a guard who is raping female prisoners. "I thought if we were going to show physical violence, it should be as chilling and horrifying as we could make it," she says. "It was the hardest show I've ever done, but very liberating, too. I called my acting coach to talk it through, and I remember thinking how lucky I am that I get to feel nervous after nine years of playing this character."

In the April 22 episode, Benson shows another kind of nerve. While she and Stabler are trying to solve the murder of a closeted gay football player's boyfriend, it's revealed that she's also been hiding something: a secret boyfriend, played by Bill Pullman. "He's a little bit older and stronger and a bit of competition for Stabler," says executive producer Neal Baer. "Please," Meloni protests. "He's an egghead. He's a reporter for the New York Ledger." Hargitay provides the backstory: "Olivia was turned down for adoption and figures if she gets married, she can be a parent. But as much as she wants stability and somebody to go home to, he's not the one. He wasn't right for her and she knew it. So she didn't tell Elliot because he's her mirror — looking at him is like looking at the truth."

Not that her short-lived love interest was particularly hard on the eyes. "If you're going to have a boyfriend, you want it to be Bill Pullman," she admits. "He was absolutely lovely and it was hard to break up, because we had just met. It was sad times, but he planted a good one on me before he left."

Meanwhile, Stabler is back with his wife and kids, including a new baby. "Elliot is going to be dealing with his 17-year-old, who is now a problem child," Meloni says. "He has gone to the dark place with his rage, and I think having nearly lost his family, his wife, his new baby, he sees life through different eyes…. "

For more on SVU, pick up the new issue of TV Guide, on sale now….