Season 4, Episode 9 - The Boyfriend Complexity
Aired November 18, 2010
Penny's dad is in town and Leonard is confused when Penny starts acting romantic with him. Penny asks Leonard to lie to her father and pretend they are still in a relationship. Meanwhile, Howard, Raj and Bernadette spend the night monitoring a telescope in the astrophysics lab, but the night takes an awkward turn when Raj starts drinking.