The long-awaited fifth season premiere of Mad Men on Sunday arrived with both hearty critical approval and a veritable theme song of its own. (Warning: Spoiler alert.) Fans and reviewers are buzzing about the "bizarre, come-hither burlesque" routine that Don Draper's new wife, Megan, delivers at the surprise 40th birthday party she throws for him in circa-1966 Manhattan. The lithe young bride, played by Jessica Pare, performed a sultry version of "Zou Bisou Bisou," a hitherto little-known French ditty. Here, everything you need to know about the strangely indelible song:

What was the scene?
Though Elisabeth Moss' Peggy warns her that "men don't like surprises," Megan invites most of Don's co-workers and their significant others to the Drapers' swanky new highrise apartment. After some strained chit-chat between the Sterling Cooper Draper Pryce contingent and other cutting-edge party guests (a black homosexual!), Megan announces that she's sufficiently drunk to offer Don her gift of song and dance, and launches into "Zou Bisou Bisou," accompanied by the live band. Soon she's offering up "full-on sex kitten" dance moves and writhing seductively in an embarrassed Don's lap. Though the guests applaud dutifully, the next day at work, Don's co-workers begin mocking Megan's shenanigans.

What's the history of the song?
Repeating the phrase "Zou Bisou Bisou" (which roughly translates as "Oh! Kiss, kiss!") no less than 15 times, it's a primo example of "yéyé," a French pop genre in which young singers exuded a faux-innocent sexuality. Megan's version is based on the 1961 recording by actress Gillian Hills, a 16-year-old "Brigitte Bardot lookalike." (Watch the video below.) The single became an international smash, particularly popular in France, Spain, and Quebec, and later showed up in the 1966 film Blow-Up, as the soundtrack to a steamy menage à trois scene co-starring Hills. The song was produced by George Martin, who later worked with The Beatles, and co-written by Bill Shepard and Alan Tew, who later composed theme music for The People's Court. An English version, known as "Zoo Be Zoo Be Zoo" was recorded by Sophia Loren as part of "an album of duets and solo songs" by Loren and Peter Sellers to capitalize on the success of their 1960 film The Millionairess, says Sean Howe at New York

What do the lyrics mean?
The lyrics celebrate the softness and gloriousness of kisses. Several key lines in the verses are about coming out from "the bushes" where "lovers glide stealthily" and expressing love "everywhere," says David Haglund at Slate. "This not only echoes what Megan herself is doing in performing the song for Don in front of so many people, it also resonates with the broader shift in sexual mores that took place in the mid-1960s." Especially knowing the history of yéyé and the image Hills presented while singing it, it's doubtful that Mad Men creator and writer Matthew Weiner could have found a "more perfect and surprising song with which to convey the sexual liberation of Megan and her generational cohort."

And we can download it?
Hours after Mad Men's premiere, AMC and Lionsgate Television Music released Pare's cover of the song on iTunes, and began selling limited-edition vinyl recordings on the show's website. Soon, says Erin Carlson at The Hollywood Reporter, it will be available on Amazon, too.

Sources: Daily Mail, Hollywood Reporter, New York, Slate