Jessica Pare gets the buzz world Zou Bisou Bisou "

The newcomer was the showstopper.

Mad Men's Jessica Paré, who arrived in Season 4, had fans buzzing after Sunday's season premiere with her character's sexy rendition of the 1961 French pop hit Zou Bisou Bisou and a steamy carpet liaison with her new husband, Don Draper (Jon Hamm).

"That was a big surprise for me. I was happy just to have a job, of course, and then I had all these incredible scenes to do. It's a huge gift," she says.

It's been a Mad Men whirlwind for Montreal native Paré, 29, seen last season as secretary Megan Calvet in a role that could have been short-lived but has evolved into series-regular status. Her character grew closer to Don during a trip to California, leading to the much-talked-about season-ending bombshell in which he proposed to her. Fans learned Sunday that the two had gotten married.

"Some people felt like (the proposal) came out of nowhere. Don's met lots of women and hasn't proposed to any of them," says Paré, whose credits include Stardom, Lost and Delirious and Hot Tub Time Machine. "Megan offers something different. She has a real joie de vivre and talent for being happy that is not the undercurrent in the rest of the characters."

After last season, Paré was up in the air about her character's fate. "I wasn't sure if I would be a part of it. Even though it did leave on a cliffhanger, all they had to do was open on a closed casket, and that would be the end of Megan."

In Sunday's fifth-season opener, Mad Men's most-watched episode ever (3.5 million viewers), Paré's Megan Draper throws Don a 40th birthday surprise party, and he is none too pleased. She caps it off with the singing performance, complete with dance moves, as a mortified Don suffers in silence. (The single is available on iTunes, with a vinyl edition to be sold on AMC's Mad Men website.)

"It was a daunting proposition, especially given that I don't dance or sing professionally," says Paré, who had choreographic training for the scene. "But it's exciting. … Any awkwardness that I felt, any embarrassment, only served to bring the scene more to life."

Don's displeasure leads to tension between the couple, an argument that turns into a romantic entanglement. "It's so powerful. There's something quite shocking in it," she says. "Obviously, Don and Megan have this great physical connection. That is part of the foundation of their relationship. Here, she's stuck in the position where she's sort of lost the upper hand and is blatantly making a play to regain it."

She was more anxious about the challenge of the scene than having to act it out in her underwear. "There was probably more coverage than a few bathing suits I own."

Paré won't give away details about what's coming up on the Emmy-winning series. Whatever happens, she will always have Sunday's tour de force.

"Where it went in the end of last season, that was enormous for me," she says. "And then coming back and having so much to do in the first episode, it's still very surreal. I honestly cannot believe my luck."

Mad Men: Don Draper Do Make the right choice?

When Mad Men's season 4 wrapped up nearly a year and a half ago, Don Draper (Jon Hamm) was on the precipice of a major life change. He had proposed to his young secretary, Megan (Jessica Paré) after taking her on a trip to Disneyland with his children.

As in seasons past, Sunday night's premiere episode has been shrouded in secrecy – with nary a plot point revealed. (Not even the year in which the season is set!)

So, if you haven't seen the season 5 premiere yet, stop reading! There are spoilers ahead …

It's May 1966, and Don and Megan are married. The Civil Rights movement is on everybody's minds and protest marches are taking place on Madison Avenue. Pete Campbell (Vincent Kartheiser) has moved to the suburbs with his wife, Trudy, and their baby. And Joan Harris (Christina Hendricks) has an infant at home – a child she fathered with Roger Sterling (John Slattery) while her husband (who thinks the baby is his) is stationed in Vietnam.

At the Sterling Cooper Draper Pryce office, the staff attempts to secure new business, specifically Heinz Beans, an account on that has Peggy (Elisabeth Moss) working overtime.

But central to the episode is Don: Much is revealed about his new home-life, how his three children respond to Megan – they seem to like her – and how the marriage and work relationship bleed together (at times uncomfortably) as she works at the agency, although no longer as his secretary. Don's ex wife, Betty (January Jones) was entirely absent from the two-hour season premiere, except in a passing mention when Don delivers the kids back to her house.

Don Hates Surprises

In a pivotal scene, Megan throws Don a surprise 40th birthday party, even though she knows he hates surprises. By welcoming his coworkers and friends, hiring a band and decorating their apartment, she has invited the outside world into their private lives. At one point, Megan serenades the birthday boy and performs a coquettish dance to Gillian Hills's "Zou Bisou Bisou," a flirty, silly come-on. The performance embarrasses Don – and causes an unsettling rift for the two of them both at home and at work.

Humiliated after the party, Don lashes out at her, and Megan is left over their apartment balcony, staring aimlessly.

At the end of Sunday's premiere, a disgusted and embarrassed Megan leaves work and ends up in their soiled apartment living room, attempting to clean up after the party. Don then comes home early, and – after she strips down to her bra and underwear – the two fall into an uncomfortable embrace and begin having sex on the living room floor. It's a chilling scene that begs the question: What's next for this couple.