The men have a mission on the episode "The Zarnecki Incursion." They must find the person who hacked into Sheldon's online game account.
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That's Katee Sackhoff and that's George Takei. Both guest star as themselves on the episode "The Hot Troll Deviation."
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Science pays off for Wolowitz on the season four premiere. He gets some solid use out of his robotic arm.
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A flashback to the nerds when they first met playing Gamecube together.
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Poor Penny attempts to go bowling as Raj and Wolowitz watch on and drool.
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Bernadette (Melissa Rauch) joins the guys for lunch at work as she begins to bond with Leonard over his work.
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Wolowitz confronts Leonard when he gets jealous over his friend bonding with his girlfriend, Bernadette.
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Who knew Wolowitz could sing and play the keyboard? Well soon Bernadette will know.
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TBBT Quotes

Penny: Here's a question-- as an alien pretending to be human, are you planning to engage in any post-prom mating rituals with Amy?
Sheldon: There are post-prom mating rituals?
Penny: Not always. Unless your date drives a van with an air mattress, then always.
Sheldon: Well, if it's part of the prom experience, then I'm open to it.
Penny: You're kidding.
Sheldon: I may be an alien, but I have urges.If Amy wants to copulate by firing her eggs into space, well, then, I will happily catch them with the reproductive sac on my upper flermin. I'm not the best at reading facial cues, but I can see that you're a little turned on.

Raj: Well, to paraphrase Shakespeare: It's better to have loved and lost than to stay home every night and download increasingly shameful pornography.
Penny: Oh... you poor baby.
Raj: What's wrong with me, Penny?
Penny: Nothing, nothing. You know, if we weren't friends - and you hadn't brought up that creepy pornography story - I'd be on you like the speed of light squared on matter to make energy.
Raj: Hey, you totally got that right. E = MC squared.
Penny: I listen. I have no idea what it means, but I listen.