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Leonard announces that the professor who was to attend a conference in Switzerland and visit CERN's Large Hadron Collider is unable to go, so he (Leonard) has been designated to be the replacement, and he gets to take one guest with him. Sheldon is pleased, assuming that he will be Leonard's guest because they have a roommate agreement that if one roommate gets invited to see the Collider, he would invite the other (Sheldon having always wanted to see it). In spite of the agreement (which Leonard had thought was frivolous), Leonard chooses to invite Penny, as the trip would coincide with Valentine's Day, their first together as a couple. Sheldon is upset and tries to pressure both Leonard and Penny to let him go instead, but Leonard insists on bringing Penny and not Sheldon, and Sheldon eventually dec...


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