Probably only second to 'The One With The Embryos' in season 4, this episode sees Monica and Rachel reclaim their apartment.
They bet Chandler and Joey again for the apartment and lose, again. However, one day when Chandler and Joey are out, Monica and Rachel switch apartments back and move all of the guys stuff into their former apartment. Chandler and Joey are initially mad, but are very willing to move back to their old apartment after Monica and Rachel agree to make out with each other for one minute.
Ross and Emily drop a bombshell on the group when they reveal that they are going to get married. Despite trying to hide it, it is obvious that Rachel is disturbed and unhappy with the decision.