In Plain Sight In Plain Sight

"What is it with babies? I don't get 'em!"

Tough-as-nails workaholic U.S. Marshal Mary Shannon (Mary McCormack) uttered those very words in the first season of In Plain Sight. Now, three years later, an unexpected pregnancy will force Mary to finally face the music and, maybe, understand motherhood during what might be her toughest assignment yet.

"Mary's not into the physical changes, she's not into the miracle of pregnancy. She has no interest in being pregnant or being a mother," McCormack tells "I don't know what the outcome will be."

Mary McCormack is pregnant in real life and on In Plain Sight

The fourth season of In Plain Sight, premiering Sunday at 10/9c on USA, will explore not only Mary's unconventional response to impending motherhood, but also how she balances the upheaval in her personal life with her demanding job.

The baby news is timed, not surprisingly, with McCormack's real-life pregnancy. The actress, 42, is expecting her third child with husband, producer Michael Morris. Since McCormack's first two pregnancies were never documented on camera — her first was hidden behind big desks on The West Wing and her second took place between the filming of the pilot and first season of In Plain Sight — she wasn't sure what to expect when she told her bosses at USA a month into her pregnancy.

"[USA Network Co-President] Jeff Wachtel and I had a nice long discussion about the benefits of playing her pregnant," she says. "He made the point and said, 'Usually in Season 4 of our shows, I love to watch our characters have to go through huge changes. Here we are presented with this huge change, and maybe we seize the opportunity. ...I think he was right."

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Despite Mary's reluctance to play the role of mother-to-be, the baby comes just as everyone else in her life begins to rely on her less and less. In the season's first episode, Mary accuses her sister, Brandi (Nichole Hiltz), of being involved in a car heist. When Mary realizes she's wrong, she also finally accepts that her sister is in a healthy relationship and ready to walk down the aisle. Meanwhile, her mother, Jinx, remains sober, and longtime partner Marshall (Frederick Weller) is in a serious relationship.

"For Mary, her identity has been as a caretaker. Her whole life she's had to clean up everybody's messes and now there are no more messes," McCormack says. "If you're a caretaker, who are you when there's no one else to take care of?"

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With Marshall in a new relationship, what's next for Mary's love life? Although she was last seen taking a spontaneous vacation with FBI agent Michael Faber (Steven Weber), McCormack remained tight-lipped on whether he'll still be around this season. "There's a little bit of a whodunit with my pregnancy," she says with a laugh. "We don't stay coy. I think viewers find out in episode 6 or 7. ... We don't string it out in what could be an annoying way."

McCormack insists she doesn't know what will happen when the baby comes, but she's ready to see Mary embrace the many changes ahead.

"I think it's time for a big change like that," McCormack says. "Ultimately, I think the audience wants and deservers to see them grow and move in different directions."