The Cushion Saturation
The Big Bang Theory Season 2 Episode 16 (03/02/09)
The episode opens with the guys playing paintball. They're stuck in a shack and under heavy fire when they decide to all bailout and leave Wolowitz to cover for them. However, shortly after the other guys leave, Leslie comes to join Wolowitz in the shack and the two of them have to find something to do to kill time while they're trapped.
Soon Wolowitz and Leslie enter into a "relationship" where the two of them continue to have sex and Leslie keeps hooking him up with special privileges at work such as new equipment and trips. However, when Wolowitz refuses to go to a wedding with her, she threatens to cut him off. Wolowitz quickly agrees to go.
Meanwhile, after the paintball game, Penny come over to the apartment and plays with one of the paintball guns and accidentally shoots a ball into Sheldon's seat. After attempting to flip the cushion, Sheldon finds the seat unacceptable, so Penny takes the cushion to be cleaned. After returning with a clean cushion, Sheldon still finds the seat unacceptable, so Leonard tells him he's be serving him Chinese food from a new restaurant for the last two years in the old restaurant's boxes. This news is so devastating to him he almost forgets about the seat, but not quite.
The episode ends with the group playing a game of paintball, now with Penny, and Sheldon ends up shooting Penny as his revenge.