'Big Bang Theory': Penny and Leonard Take a Major Step Forward (Video)

"The Big Bang Theory's" Kaley Cuoco and Johnny Galecki

[Warning: This story contains spoilers from Thursday's "The 43 Peculiarity."]

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CBS' The Big Bang Theory took one giant leap for Penny and Leonard during Thursday's episode.

After months years seasons of on-again-off-again romantic tension between lovably loyal Leonard (Johnny Galecki) and the girl across the hall Penny (Kaley Cuoco), the latter finally muttered the three little words that she's thus far been unable to say to her boyfriend in six seasons.

That's right, Penny finally told Leonard she loved him -- but there was (naturally!) a catch.

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During the episode, new (again) student Penny had a classmate with a sexy London accent over to study, bringing out Leonard's insecurities and a bit of the crazy after Leonard none-too-slyly told Cole (Bones' Ryan Cartwright) that Penny's boyfriend was a gang member (the Scientists!).

After being caught in the act, they retreat back to their respective apartments, caught in an awkward moment.

Later, Penny confronts Leonard about it in the hallway for an explanation as he confesses that no matter where they go, guys are always hitting on her. "It's all in my head, it's my problem, not yours," he tells her.

"Leonard, why do you always do this? Listen to me, you are the one I'm with, you know I love you. So will you please relax because you're driving me crazy!" Penny tells a stunned Leonard as she quickly realizes what she just said and marking the first time she's ever told him as much.

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It's a monumental event for the series, which last season saw Howard (Simon Helberg) and Bernadette (Melissa Rauch) tie the knot, while Sheldon (Jim Parsons) and Amy (Mayim Bialik) took a big step with a pivotal hand-holding moment in the season finale.

Of course if the couple's history is any indication, it won't all be smooth sailing as there's already one very clear hurdle in Sheldon's assistant Alex (Margo Harshman), who clearly has eyes for Leonard.

Check out the moment below and hit the comments with your thoughts on the "aww" moment of the season. The Big Bang Theory airs Thursdays at 8 p.m. on CBS.

Email: Lesley.Goldberg@thr.com; Twitter: @Snoodit