Sheldon's regular barber goes into the hospital. Because Sheldon is obsessive, he has gone to the same barber since arriving in California. (His mother told him that she'd provided his "hair cut records" to this new barber.) As Sheldon continues to go without his haircut and nothing horrible happens, he begins to realize that the careful structuring he's built around may have been for naught. As part of his new habit of embracing the chaos of the world, he decides to take up the bongos. He finally consents to a haircut from Penny.
Meanwhile, Howard arrives at NASA for his training as a payload specialist in a space mission, but the training might be just too much for him. Concerned about his well-being, Bernadette comes to the training facility to stay with him, only to discover that his mother is already there to help.
This Episode's Science
Zero Gravity Training - While being trained at NASA, Howard goes through zero gravity training. This training works by a plane going up in the air in a steep parabolic path. While the plane is descending it's moving fast enough that the inhabitants are essentially in free-fall, so they can't feel the effects of gravity (even though gravity is still obviously affecting them). The Zero Gravity Corporation conducts private flights so that non-astronauts can experience the sensation.
Physicists Playing Musical Instruments - Physicist Richard Feynman was known as a "curious character," in part because one of his hobbies was playing bongos. This is a little more flamboyant than the violin hobby of Albert Einstein, but it's the one Sheldon apparently decided to emulate in this episode.
Penny: What the hell?
Leonard: Hi, Penny. Guess what? Sheldon got bongos!
Penny: [to Sheldon] Why did you get bongos?
Sheldon: Richard Feynman played the bongos. I thought I'd give that a try.
Leonard: [to Penny] Richard Feynman is a physicist--
Penny: Leonard, it's 3 o'clock in the morning. I don't care if Richard Feynman is a purple leprechaun who lived in my butt.