'Big Bang Theory' Sneak Peek: Penny and Leonard Have a Spat -- and Sheldon Crashes the Party! (Exclusive)

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Sheldon Cooper has the worst timing.

In ET’s exclusive sneak peek from Thursday’s episode of The Big Bang Theory, Sheldon (Jim Parsons) unintentionally crashes Leonard (Johnny Galecki) and Penny’s (Kaley Cuoco) innocent spat about a documentary they watched the night before. (Spoiler: Penny wasn’t a fan!)

When Leonard makes the same joke about the doc, Penny is not amused. “I already pretended to laugh at that joke once, do I have to do it again?” she bemoans, but eventually caters to Leonard’s wishes.

RELATED: Kaley Cuoco Says Penny and Leonard Might Be Getting the Baby 'Itch' on 'The Big Bang Theory!'

Seconds later, Sheldon interrupts their lovers’ quarrel and asks Leonard to drive him to work, but gets mysteriously coy when he’s asked why he can’t make it to a Tesla coil exhibition with the crew. “I couldn’t tell you even if I wanted to,” Sheldon says.

Last week, Galecki exclusively spoke with ET about the episode, which sees Sheldon going behind Leonard and Wolowitz’s (Simon Helberg) backs when he goes to work with the military. When they find out, they’re not pleased.

“That’s a fun episode about narcissism and how it can hurt the people around you -- not because you think yourself better of them, but because they are afterthoughts when it serves you. Which Sheldon is really good at,” Galecki says. “It’s a topic we’ve hit on before because it’s [prevalent] in our culture and our relationships, unfortunately. This is from a much more mature, professional perspective.”

The Big Bang Theory airs Thursdays at 8 p.m. ET/PT on CBS.

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