Katharine McPhee Flashes Major Sideboob in Plunging Black Dress at Golden Globes

Katharine McPhee arrives at the 70th Annual Golden Globe Awards. Steve Granitz/WireImage

Talk about your not-so-basic black. Smash star Katharine McPhee looked, well, smashing at Sunday's 70th annual Golden Globe Awards.

Walking the red carpet at the Beverly Hilton Hotel in a plunging Theysken's Theory gown with a thigh-high slit, the actress made quite an impression. Even Ryan Seacrest was temporarily at a loss for words.

Flashing some serious cleavage (plus sideboob!) and a lot of leg, McPhee told the E! News host that she wanted to wear something "sexy," to which he replied simply: "You've accomplished sexy."

Indeed, the American Idol alum — who accessorized her gown with Neil Lane jewels and an Oroton clutch — has come a long way from the plain-Jane she pretended to be when she first burst onto the scene several years ago.

"On Idol, I was very strategic with my look," she told Lucky magazine in a recent interview. "I specifically wore very little makeup, did very little with my hair, I wore beat-up jeans and a simple jacket, and I covered my body up because I knew that viewers like seeing transformation…I wanted to create that story — actually to push the story that I had no idea about fashion — which was not actually true."

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