Yes, That Was Linda Cardellini on 'Mad Men'

Linda Cardellini in "Mad Men" and "Freaks and Geeks." (AMC, NBCU Photo Bank)

[SPOILER ALERT: This article discusses key plot developments from tonight's season premiere of "Mad Men." If you don't want to know what happened, stop reading now.]

At the very end of tonight's "Mad Men" season premiere, we learned that Don Draper is once again hopping into bed with a new woman… not exactly a shocker. What was shocking, though, is who he was hopping into bed with: his neighbor's wife Sylvia Rosen, played by none other than TV veteran Linda Cardellini.

To be honest, we didn't even recognize Cardellini at first. (Those heavy '60s eyebrows threw us off.) She made her "Mad Men" debut late in the two-hour premiere, as Don and his (oblivious) wife Megan celebrated New Year's Eve in their swanky apartment with a pair of neighbor couples, including Don's friend Dr. Arnold Rosen and his wife Sylvia.

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At first, we thought Don might end up sleeping with the drunk blonde who was blatantly flirting with him in front of her husband. But in a classic bit of "Mad Men" misdirection (one that cleverly mirrors the show's pilot), we discover that it's Sylvia who's lounging around between the sheets with Don these days, as the two share an intimate bedroom scene. (And she's the one who lent him that copy of Dante's "Inferno" he was reading on the beaches of Hawaii as the episode opened.) Sleeping with your neighbor's wife… a dangerous game, Don Draper.

Check out the reveal of Don and Sylvia's tryst from tonight's "Mad Men" right here:

As TV fans, we remember Cardellini best as honors student-turned-burnout Lindsay Weir from NBC's single-season gem "Freaks and Geeks." (Judd Apatow, if you ever want to launch a Kickstarter for an "F&G" ten-year high school reunion movie, count us in.) All of Cardellini's fellow "freaks" -- James Franco, Seth Rogen, Jason Segel, Busy Philipps -- went on to have higher-profile careers than her, but she landed on her feet, too. She played Velma in the big-screen "Scooby-Doo" movies and joined the cast of "ER" in 2003, playing nurse Samantha Taggart for six seasons. And now she's in bed with Jon Hamm... not too shabby.

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And she's not the only familiar face we spotted on the "Mad Men" premiere. James Wolk, star of the woefully short-lived Fox drama "Lone Star," played upstart SCDP account man Bob Benson, who managed to annoy both Don and Ken with his bald-faced attempts at climbing the corporate ladder. We've always thought Wolk deserved a better shot at TV stardom than the two episodes "Lone Star" got… and he does look perfect in those retro "Mad Men" suits. Here's hoping he'll be a thorn in Don's side for weeks to come.