Howard and Leonard vie for the affections of Sheldon's visiting twin sister, Missy. Raj participates in a trial for an experimental drug that allows him to overcome his selective mutism.
- Big Brother Instinct: As the one-upmanship escalates, Sheldon becomes more protective of Missy.
- Early Installment Weirdness: Missy talks about a time when Sheldon built a robot arm to try and keep her from going into his room. As later shown on Young Sheldon, Sheldon and Missy shared a bedroom when they were kids. Though it should be noted that the incident took place when they were in second grade, which would have taken place before the Cooper family moved to the house in Medford as shown in Young Sheldon (as the family previously lived in Galveston and had to relocate due to George being caught up in a recruiting scandal at the school he coached at), so it's possible that the twins had separate rooms in their old house.
- Groin Attack: How Missy corrects Sheldon.
- Insufferable Genius: Missy confirms that Sheldon was one even as a child.
- One Head Taller: Sheldon's usually the tallest, but Missy also towers over all the guys.
- Ordered Apology: What Missy requires. Usually with the aforementioned Groin Attack.
- Special Odd Hand: A mild version. When Raj first takes the drug, his hand starts waving on its own.Raj: They mentioned there might be side effects.