"The One With The Red Sweater" is the second episode of the eighth season of Friends, which aired on October 4, 2001.
Checking out of the wedding suite from the hotel is no happy thing for Monica, who is actually depressed that her wedding day is over. Her and Chandler's honeymoon is in three days, and the photographer's pictures will take weeks to get developed. Chandler, however, reminds her that they have the pictures from the disposable cameras at the tables to look at. Unfortunately, Chandler can't find the cameras, so he asks Ross to help him out. Ross, who checks out after the others, points out to his brother-in-law that another wedding reception is about to take place in the same hall he and Monica had theirs last night. The two buy new disposable cameras and dress up for a wedding reception they're not even invited to, hoping that Monica will never know about this.
Monica, meanwhile, is flipping out because she promised Chandler they would open their wedding gifts together, but can't as he's still not home. When her curiosity gets the best of her, she opens a tiny one which contains a salt shaker. Hoping to complete the set, she opens the second one, which is roughly the size of a beach ball. She continues opening presents with uncommon ecstasy. Phoebe scares Rachel from telling the father when she remarks how her confession will change the father's life completely, something which Rachel doesn't want as she's ready to raise her child by herself.
Rachel is still intent not to tell who the father is, but when Joey suspects something, Phoebe sticks with her story from the wedding that she is pregnant. Joey is incredibly happy for Phoebe but equally sad that she'll be raising her kid without a father, so he proposes to fix things - by proposing to Phoebe, who accepts. Monica tells Joey that it's Rachel who's pregnant, and how she and Phoebe know about this but don't know anything about the father. This sparks something in Joey's mind, who at his apartment, finds a red sweater which he has had for a month and believes is the father's. This, in turn, is a revelation for Phoebe, who exclaims how she knows who the father is.
At the reception, Ross and Chandler take tons of pictures where they try to integrate in the wedding, including one with Ross making a toast on stage (ending up warning a guest that his '95 blue-green LeSabre to move his car before it gets towed) and Chandler kissing a totally unknown bride, a picture after which Ross and Chandler run for their lives. Phoebe drags Rachel to Central Perk, where she has arranged for the father to be. Rachel, still unsure to tell the father, enters, and finds Tag in front of her. Phoebe pulls out the red sweater Joey found at his apartment, but it turns out it's not Tag's, who unzips his jacket to reveal his red sweater. Deeply embarrassed, Phoebe leaves the two alone, and Tag takes the opportunity to hold her hand try and get back together with Rachel, who loses no time in telling him that she's bearing child. Tag drops her hand and remains speechless until she gives him the permission to leave.
At the apartment, Monica has finished opening all of the gifts (including Joey's parents', which according to Joey, "does something to salami"). Joey, who now knows that Rachel is pregnant, proposes to her in front of Phoebe, who does not take kindly to this. Unlike Phoebe, however, Rachel refuses to marry Joey. The group is joined by Chandler and Ross, who show the pictures to Monica. Surprisingly, Monica pulls the real disposable cameras out of one of her bags. Although she's not amused to see Chandler kissing another woman, he's quite annoyed to find all the presents opened. Determined to start the wedding on the right foot, however, the two call it even. Ross is about to leave, when he spots the red sweater. He then claims that it is his, much to Monica and Phoebe's shock. He then leaves as Monica and Phoebe realize that he is the father of Rachel's baby. Joey, slow as ever, takes a minute to work this out, but is just as shocked as the others.