The Big Bang Theory: The Large Hadron Collision / Season: 3 / Episode: 15 (2010)

Leonard announces that the professor who was invited to a trip to see CERN's Large Hadron Collider in Switzerland was unable to go, so he (Leonard) was designated to be the replacement to go to Switzerland, and he gets to take one guest with him. Sheldon assumes that he would be Leonard's guest because they have a roommating agreement that if one roommate gets invited to see the collider, he would invite the other. In spite of the agreement, Leonard chooses to invite Penny, as the trip would coincide with Valentine's Day, their first together as a couple. Sheldon tries to pressure both Leonard and Penny to let him go instead, but Leonard insists on bringing Penny and not Sheldon. The night before their scheduled trip, Penny falls ill, so Sheldon is designated to accompany Leonard. However, Sheldon immediately falls ill too, which he suspects he caught when he hugged Penny a day before. Raj therefore accompanies Leonard, and they arrive in Switzerland to a hotel room stocked with champagne, roses, and chocolates.