5x14 "The One Where Everybody Finds Out"

It's a sad day as Ugly Naked Guy is moving out, and Ross decides to apply to sublet his apartment. Phoebe finds out about Monica and Chandler after seeing them through Ugly Naked Guy's window and although Joey is happy that now they can just confront them, Rachel and Phoebe instead decide to pretend Phoebe is romantically interested in Chandler, so as to make them confess. Monica and Chandler realize something's up so they play along. Ross finds out that quite a few people are interested in Ugly Naked Guy's apartment and goes to extremes to win him over. Eventually the competition between Monica, Chandler, Phoebe and Rachel comes to a head, and Chandler and Monica admit that they love each other. Despite this, they still wish to keep their relationship a secret from Ross ... but it isn't long before he finds out too.