Fed up with Sheldon's rude behavior (he spoiled some major Harry Potter plot twists), Leonard decides they can't live together anymore. When he decides to move in with Penny, she isn't ready to move their relationship to that level. Sheldon is then confronted with Amy deciding to fill the roommate void. Afraid to own up to the fact that he doesn't want to move in together, Sheldon tries to blame Penny for denying her roommate status, causing the web of lies to fall apart.
Meanwhile, Howard asks Raj to visit with his Mom while he's out of town. Raj goes by for dinner, but ends up spending the night in Howard's room, becoming a stand-in for Howard's mother's parenting needs.
This Episode's Science
Amy: [Upon being told she can't move in with Sheldon.] I did everything just the way you like it.
Sheldon: You did.
Amy: Then what the hell, Sheldon? We have been going out for over two years, and I have been nothing but patient with you. I watch your dopey space movies. I signed your ridiculous contract. I even stopped wearing lip gloss ’cause you said it made my mouth look too slippery. I am the best girlfriend you’re ever gonna have. You give me one good reason why I can’t live here.
Sheldon: It’s Penny’s fault.
Sheldon: She doesn’t want to live with Leonard, so he has to come live here again. She’s the snake in our garden. She’s the reason we can’t be happy.