The whole premise of TBBT is what happened to Sheldon, Leonard and their nerdy friends when Penny moved in across the hall. What if Penny didn't move in? Everything belongs to Lorre, Prady and friends.
The story opens as Penny is being shown apartment 4B
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Sheldon, Leonard, Raj and Howard came out of 4A as Penny came out of 4B with the building manager. Leonard, Raj and Howard stopped and stared, Howard broke the silence by saying "hello pretty lady" before Sheldon said "come on gentlemen, it's new Comic Book night." The other 3 reluctantly followed Sheldon down stairs.
After the men disappeared down the stairs Penny finally spoke.
"Who were they?"
"If you move in here two of those gentlemen would be your neighbours. Dr. Cooper and Dr. Hofstadter are scientists who work at Caltech."
"I see, well thanks for showing me the apartment, I have a couple more apartments to view today. I will let you know if I still want the apartment by tomorrow."
"No problem."
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That evening returned to Bernadette's apartment, where she was temporarily staying after breaking up with Kurt.
"Hi Bernie, good news, I have found a new apartment after looking at five different apartments today."
"Well done Penny, where is it?"
"It's just round the corner from the Cheesecake Factory. I did see one on Los Robles that looked good but the rent was at the top of my price range. Also there were some rather creepy looking nerds living across the hall, imagine having people like that as neighbours?"
"I have enough time with people like that at university, good to deal with normal people most of the time."
"Anyway, I am moving at the weekend, thanks for letting me stay here. Get your dancing shoes on, let's have a girls night out to celebrate."