'Til Death.

It starts as a fairly generic sitcom. Newlywed couple buys a home in a new city with their neighbors being an older couple that is kind of over things. Eventually the show drops the newlywed couple, focuses on the older couple, and becomes a sitcom about the older couple dealing with their still at home kid and their loser boyfriend. By the end the loser boyfriend thinks he is in a sitcom, frequently comments on his wife being played by a different actress, which was played by multiple actresses by that point, and it just goes from there.