The captain tells the squadron that he is expanding it to four members, which concerns all of the lieutenants. Lt. Purcell states that four is an unlucky number because it's not prime. He then defines a prime number.

In this clip, Charlie lectures about the Riemann Hypothesis in his graduate number theory course.

Nick explains what a prime number is and determines that the serial killer commits a murder on dates in the month that are prime numbers.

What is the "best" number? There's only one according to Sheldon! In this clip, Sheldon explains what the best number is and why it is the best number.

The team listens in to communications coming from outer space and notices a pattern: each signal represents a prime number. They determine that the signal is coming from the star, Vega. After recording the sequence, the team displays on the computer screen that every prime number between 2 and 101 has been sent to them.