Since neither of them have plans for the evening, Penny asks Leonard if he'd like to go to the movies with her. They haven't gone out together as friends since stopped dating, which is a very different experience. Freed from the motivation of trying to end the evening romantically, Leonard begins standing up for doing the things that he wants to do, instead of constantly caving to Penny's whims. This backfires when, since they aren't going on a date, Penny decides to begin hanging out with another geeky guy wearing black glasses. He begins talking to another woman, but when Penny's new geek leaves, she wants his attention back, resulting in a fight that ruins the evening for both of them.
Back at the apartment, Sheldon is suffering from intense ornithophobia, brought on by a bird who seems. This initially ruins his evening watching Star Wars with Howard and Raj, but then when he gets Bernadette and Amy to come over, he instead bonds with the bird and overcomes his phobia, only to be abandoned by the free-spirited creature.
This Episode's Science
Ultrasonics: While trying to scare off the bird on his windowsill, Sheldon (with Howard's help) creates a device to emit an ultrasonic pulse. The goal is for this high-frequency sonic wave to cause the bird to leave. (See more on the Mathematical Properties of Waves.)
Howard: Alright, Sheldon, your bird death ray is ready.
Sheldon: It's not a death ray. It's just a little ultrasonic blast to scare him off. Trust me, if I had a death ray, I wouldn't be living here. I'd be in my lair, enjoying the money the people of Earth gave me for not using my death ray.
Amy: [Having been called, with Bernadette, over to help Sheldon] Sheldon, what do you expect us to do?
Sheldon: [cowering in the doorway] You're biologists. Biology is the study of living things. That's a living thing. Get cracking.
Bernadette: I specialize in microorganisms and Amy studies brains.
Amy: Yeah, neither of us minored in bird shooing.
Sheldon: Oh, come now, your undergraduate work must have included a varmints and critters class.