When Charlie's girlfriend, Chelsea, comes down with the flu in the middle of the night, Charlie follows his gut instinct and offers to take her home so she can recover - and he can avoid getting sick. She is clearly put off by his proposition and in an effort to prove his love and commitment to her, he asks her to stay with him. Alan tells him that by asking Chelsea to marry him, Charlie agreed to take her in sickness and in health. Charlie goes back to bed, and when he wakes up he sees a frightening sight. His beautiful fiancee is gone, replaced by a very sneezy, sniffly, stuffy, leaky Chelsea. And so begins Charlie's humiliating efforts to make her feel better, and loved.
Chelsea's too ill to move, and sends Charlie on a mission to buy a variety of medicines - and tampons. He overindulges in Chelsea's cough syrup, its effects buoyed by bourbon, and convinces Alan to drive him to Chelsea's apartment, where he is supposed to pick up clean clothes and take care of her cat, Sir Lancelot. Once there, he pays Alan to clean up Sir Lancelot's litter box - filled with "mountain lion-sized" feces.
While Charlie is out, Berta comes in to take care of Chelsea, but makes her promise she won't tell charlie. She's worked hard to make Charlie expect so little of her.
When Charlie gets home, he feels sick as well, and that night has flu-induced dreams that a human-sized Sir Lancelot is using his toilet. He doubts whether he will survive the night.
In the meantime, Jake is dating a girl who he's using for help with his math homework. We learn she is 9 years old; to Alan's chagrin, Jake and the 9-year-old are in the same math class and no, she is not an advanced student.