Picking up from Season 3's cliffhanger finale, Ross is revealed to have entered Rachel's room, ultimately choosing Rachel over Bonnie. As he enters the room, he sees both women. Bonnie has shaved her head, and it has consequently gotten sunburnt, Rachel is helping her apply sunblock onto it. Bonnie leaves the room, and after sharing a kiss with Rachel, Ross goes to break up with Bonnie. When he returns to Rachel's room and informs her that he has ended his relationship with Bonnie and she has left, he is bemused to then learn that while he was doing this Rachel had written him a letter that is eighteen pages (front and back) long. Understandably tired after breaking up with Bonnie, Ross falls asleep at the kitchen table reading the letter, and when he wakes up the next morning, he lies and claims that he has read the letter thoroughly when he has not finished it. Believing him, Rachel asks him "Does it?" Unsure of what she wants him to say, Ross takes a guess and says "It does." It immediately becomes clear that this is exactly what Rachel was hoping he would say - she is overjoyed and they get back together. When he later gets a chance to actually read the letter however, he discovers that Rachel wants him to take full responsibility for all that went wrong in their relationship, and is outraged.
At the beach, Monica and Chandler are still debating why he should be her boyfriend, when she gets stung by a jellyfish. Joey remembers a documentary from the Discovery channel that suggested urinating on a jellyfish sting to make the pain go away. He "steps up" to help her, but gets stage fright and cannot go through with it, so Chandler is forced to do it instead. The three are traumatized by this event and though initialy unwilling to admit to the their curious friends what the matter with them is, eventually give into the group's pressure when they ask what happened and confess.
Phoebe is shocked to learn the truth about Phoebe Abbott being her birth mother, and is very angry at her for never trying to track her down. She leaves her house saying that she never wants to see her again, but Phoebe Senior finds her at the Central Perk. Although her anger is still bubbling for her birth mother, Phoebe realizes that Phoebe Senior is her family and tries to get a rapport going on, albeit with some difficulty.
When the friends return to the city, Chandler, Monica and Joey confess what really happened at the beach. Ross and Rachel enjoy their reunion after four months apart and have sex, but Ross is privately still angry about the fact that Rachel expects him to take full responsibility for all that went wrong in thier relationship, and is refusing to take any responsibilty herself despite the fact that she was the one who suggested that they "take a break" from their relationship (which is what caused Ross to have sex with Chloe, interpreting this suggestion as a break-up.) Afterwards, Rachel infuriates Ross by telling him how happy she is with how much he has "grown". The camera switches to Monica, Joey and Chandler hearing Ross yell, "WE WERE ON A BREAK!" and storming out of Rachel's bedroom, correcting the grammar in her letter as he leaves. Rachel responds with, "Just so you know, it's NOT that common, it DOESN'T happen to every guy and it IS a big deal!", implying that Ross had suffered with erectile difficulty and she had lied to reassure him. This results in Chandler emerging from behind the front door and exclaiming, "I knew it!"
The credits scene shows Monica and Chandler playing gin rummy. Chandler still refers to him and Monica as a potential couple, and Monica admits how he's sweet and smart and she loves him as a friend, but he will always be the man who urinated on her.