Sasha Alexander and Angie Harmon

You know there's a problem when the corpse starts laughing. All morning on the set of Rizzoli & Isles, Angie Harmon and Sasha Alexander have been struggling to get through a morgue scene. The cadaver on the slab has a zipper-like slash across the throat, but it's ­ Alexander who looks like death. She's been fighting a cold and losing.

"Man! Can someone get Sasha a Kleenex?" Harmon says to hoots and giggles after Alexander derails another take with a sneeze. The dead woman's shoulders start jiggling. ­ Alexander tilts her head back in fluish exhaustion and says, "I should probably get stock in Kleenex."

At least the show is healthy. TV's hottest crime-solving duo enters Season 4 as the third-best-rated ­series on basic cable. Rizzoli & Isles averaged more than 7.5 million viewers on TNT last season, capped by a finale that, come to think of it, had some major health issues of its own.

After serious reflection (and pressure from her biological mom, Hope), Maura Isles, the erudite, fashion-­forward Boston medical examiner played by Alexander, agreed to give up her kidney anonymously to save the half sister she barely knows. Jane Rizzoli, Maura's best friend and tough-talking colleague in the Boston Police Department, tried persuading her injured-soldier boyfriend, Casey (Chris Vance), not to go through with potentially life-threatening spine surgery. Meanwhile, Jane's kid brother, Tommy (Colin Egglesfield), Tommy's baby, T.J., and a detective had to be dug out of a collapsed building.

"[When the show returns,] three months have elapsed, the dust has literally settled and everybody's out of hospital gowns," says Janet Tamaro, a veteran TV reporter who in 2010 developed the series, based on the best-selling crime novels by Tess Gerritsen. "I didn't want Jane starting the season pacing the corridor asking, 'Are they going to live?'"

Tamaro's office on the Paramount lot in Hollywood is dotted with relics from a show that's become a pop-culture staple. Among them: a still from the Jeopardy! episode that featured a Rizzoli & Isles question; gifts and letters from fans; and patches representing various police departments. "The day Tina Fey mentioned us on 30 Rock, I was finally validated in my teenager's eyes," Tamaro says.

Whether the series can maintain its firepower is as big a question as which Hermès bag Maura will show up carrying next in the lab. Ratings, though certainly strong, have never reached the heights they did the first season, which explains why, in Season 4, Harmon says, "we're leaning toward where the ­pilot was, as far as getting a little darker, more dramatic, more weighty."

The first episode of Rizzoli & Isles concerned a gruesome murder masterminded by the Surgeon, a Hannibal Lecter type who broke out of jail and came after Jane. The only real hint of the humor to come was the scene in which Maura and Jane tasted cat food. "The ping-pong girly banter is so much fun to do," Harmon says, "but what's even richer are the ­ moments when you go, 'Whoa, these ladies are in trouble.'"

The major drama this season (skip ahead if you're spoiler averse) is the pending murder trial of Maura's biological dad, mob boss Paddy Doyle (John Doman). ­After being shot by Jane and FBI agent Gabriel Dean (Billy Burke) last season, Paddy ended up in prison on multiple murder charges. That's when the situation got truly dysfunctional. Jane figured out that Paddy faked baby Maura's death and gave her to the Isles family to raise for her protection. At Maura's phony headstone, Jane unearthed a book with the names of the dirty cops and city officials who were working for Paddy.

"The whole city is waiting to see Paddy hanged, but he's still pulling maneuvers from prison," Alexander says after her cold meds kick in. It also turns out that Hope (played by Alexander look-alike Sharon Lawrence) has been lying all these years about not being in touch with Paddy. "Hope and Paddy are in contact and involved, which makes Maura say, 'Oh, that's just great. Hope was holding on to information that's prosecutable. Now she's going to jail, too?'"

Actually, it's Maura who heads to the clink. In the third episode, she becomes the prime suspect in a homicide after her shaky alibi puts her at the scene of the crime. Jane must juggle her loyalty for Maura while also staying true to her cohorts in blue. "We work it out, but Maura definitely gets knocked around behind bars for a while," Harmon says. "She's not exactly cut out for jail, which makes it really fun to watch Sasha do those scenes. We both had a blast."

The rapport the actors have on camera is apparent off screen, too, even when half the duo is hacking and wheezing. "If I start to not make sense, just hit me," Alexander tells Harmon as they give another scene a go. "Got it," Harmon says. "Let me know if you need a spittoon."

For more with Rizzoli & Isles, pick up this week's issue of TV Guide Magazine, on newsstands Thursday, June 13!

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