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Leonard's Biggest Secret On The Big Bang Theory

Between its 2007 premiere and its 2019 finale, "The Big Bang Theory" aired a whopping 279 episodes and became one of the most popular sitcoms to ever hit TV screens. For better or worse, the series also brought geek culture to the forefront of the pop culture zeitgeist, with its tale of loud and proud nerds Leonard (Johnny Galecki), Sheldon (Jim Parsons), Howard (Simon Helberg), and Raj (Kunal Nayyar) delving often into the realms of all things nerdy. Along the way, that motley crew of comic book lovers got themselves into and out of all manner of kooky adventures and embarrassing entanglements.

As is often the case with sitcoms, most of those scenarios are brought about on "The Big Bang Theory" either out of spitefully silly interpersonal shenanigans or some character withholding a vital bit of information from their cohorts that might otherwise have helped avoid some sticky situation or another. In fact, keeping secrets became a bit of a narrative staple over the show's 12-season run. While Leonard didn't trade often in keeping secrets of his own, he let a left-field doozy slip late in Season 8. And this big bang of a secret very nearly upended his impending nuptials to longtime love Penny (Kaley Cuoco).

Leonard's brush with infidelity was a major secret

Leonard reveals his biggest secret during the series' Season 8 finale, doing so just minutes before he and Penny are set to tie the proverbial knot. And just for a point of reference, the couple's road to that fateful moment has them enduring a complicated relationship that is decidedly on-again, off-again in nature, with the Season 6 finale finding Leonard and Penny very much on again. They are also on the verge of spending some time apart, with Leonard jetting off to the North Sea for a research expedition.

It's revealed in the Season 7 premiere that Leonard has been having a great time partying with his fellow scientists while away. Once he returns, however, things get right back to hot and heavy between him and Penny, remaining so until the pair decides to take their relationship to the forever level. But just before they say "I do," Leonard admits to Penny that he'd actually had a brief romantic encounter with one of his shipmates years prior that resulted in a kiss.

It's safe to say fans were every bit as shocked by the character's admission as Penny. Thankfully, she's quick to forgive and forget, and the pair goes through with their Vegas wedding. And although that was hardly the last of the unexpected Leonard infidelity plotline, it was probably best he let that secret loose when he did.