With Stephanie spending most of her nights at the house, Sheldon states that Leonard's girlfriend has officially moved in according to their roommat bylaws.  Leonard denies it, but when Penny comes over (and Stephanie meets her for the first time), she shows Leonard that Stephanie has already moved her stuff in... they're living together!

After some itchy pants and realizing that he's living with Stephanie, he thinks the relationship is moving too fast and talks to Penny about it.  She tells him to go talk to Stephanie and he does.  When Leonard tries to tell Stephanie his feelings, she seduces him instead.  Penny sends him back in.

When Leonard tries to talk to her again, she seduces him yet again.  This time, after sex, he tries to talk to her and she begins ramble about previous breakups before he can say anything and just leaves it as "I Really Like You."  When the guys convince him to try and text her to move out, he does and gets back a text and runs off to sleep with her.

Sheldon, meanwhile, spends the whole episode suffering from hypochrondia and annoying Stephanie through the episode.  Stephanie eventually tricks him into thinking his larynx is swollen and he needs to stop talking.  Sheldon switches to using a computer to do his talking.