The Maternal Congruence

Summary
A visit from Leonard's mother, Dr. Beverly Hofstadter (Christine Baranski), delights Sheldon and horrifies Leonard. Penny is upset because Leonard didn't mention his mother's visit to her, but that is nothing compared to when Leonard learns that Sheldon knows more about his own mother than Leonard does. Leonard learns about his parents' pending divorce, his mother's surgery, and the death of his dog all after Sheldon had known. Beverly questions Howard and Raj about their ersatz homosexual relationship (alluding to her last visit in The Maternal Capacitance). Penny and Beverly go out drinking, and when they get back to the apartment, Beverly kisses Sheldon, but at the end of the episode, they all decide to keep that a secret from Leonard.

Title reference: Leonard's mother visiting him.