Season 6: Episode 22 - Sir Lancelot's Litter Box

Sir Lancelot's Litter Box: Chelsea and Charlie get into a huge fight after the lease on her apartment ends and she suggests she move in to his place, even though he still wants his own space. After some soul searching and a chat with Alan, Charlie realizes he was wrong and asks Chelsea to move in with him. However, when she gets there she changes things all around, from the shower curtains to the salt and pepper shakers, which freaks Charlie out even more and another fight ensues. Alan then helps Charlie smooth things over with Chelsea again, only to discover that now that she's moved in, they are using his room as a storage closet. Ultimately, Charlie solves the problem by secretly renting Chelsea’s old apartment for himself.