Howard and Raj become intrigued when they realize that Sheldon vanishes every day at 2:45. When they follow him, they discover that he locks himself in a basement room during this time, so they begin to investigate. In the room, they find the number 43 written on a chalkboard, but are uncertain what the number could mean. As they attempt to uncover the secret of the number 43, they are positioned for a startling revelation ... and a classic Dr. Sheldon Cooper Bazinga! moment.
Meanwhile, Leonard becomes concerned about the time that Penny is spending with her male study partner at the community college. His attempt to intimidate the guy away goes awry when he's caught by Penny. The subsequent emotional exchange reveals a previously-unspoken truth about their relationship.
Another intriguing plot point in this episode is that Leonard again has a discussion with Sheldon's research assistant, Alex, and does not realize that she is flirting with him.
This Episode's Science
Wormhole - Sheldon begins the episode drawing a doodle of a wormhole, a hypothetical tear in the very fabric of spacetime, which comes back up later in the episode as an element of his prank on Raj & Howard. A wormhole is a staple in science fiction, because it one of the only scientifically plausible means of getting information between two points faster than the speed of light, which also provides a potential means for time travel ... or, as Sheldon suggests, access to a parallel universe.
Neutrino - At one point, Leonard mentions that he's building crystals for part of a neutrino detector that he's working on.
Numerical Encryption - When trying to discover the meaning of the cryptic number 43, Raj points out that it's a prime number. Computer encryption systems use prime numbers as the basis of the encryption algorithm, although these are typically built upon numbers much larger than 43.
Raj: [nodding toward Sheldon, who is doodling on a napkin] What are you drawing over there?
Sheldon: It's a hypothetical containment for a frisbee-sized wormhole that could serve as a portal to a parallel universe.
Howard: Oh, you silly doodlebug.
Leonard: You know, a lot of scientists believe that making contact with other life forms would probably not end well for us.
Sheldon: It's a frisbee-sized wormhole, Leonard. You could block it with a frisbee. Calm down.