Rachel, (played by Jennifer Aniston in a really tight shirt and a skirt with less fabric than your average napkin), and Ross, this kind of shy but really sweet paleontologist (i.e. dinosaur dude) have had, at one point or another, crushes on each other. But due to zany sitcom screw-ups, nothing ever happened between them. (Awww...)
Last week, Rachel told Ross that nothing was going to happen between them. Ross was hurt. Wounded. Bleeding out of ever major orifice and puking black bile, figuratively speaking.
Then, Ross's sister Monica (played by Courtney "Alex P. Keaton was too short for me anyway" Cox) digs up an old videotape of her high school prom. She pops it in any plays it for the entire gang (which also includes Chandler, Joey, and Phoebe who won't be discussed much here since they weren't all that pivotal to last week's plot, but who are all very nice people in their own right, and if you ever see them on the street, offer to buy them a drink.)
So in the tape, we see Monica (who was much plumper) and Rachel (pre-nose job) getting ready to go to the prom. Ross, who is Monica's older brother was also hanging around the house. Apparently, even then, Ross had a crush on Rachel.
Suddenly, tragedy strikes! Rachel's date does not show up! Ross' parents convince him to go with Rachel. So, without Rachel knowing, Ross goes upstairs and puts on his father's tux, grabs some flowers, and prepares to go downstairs to go to the prom with Rachel. But as he starts to head down the stairs, we see that Rachel's date has arrived and they're already out the door. Ross is crushed. The entire studio audience is weeping. I pretended I had something in my contact lens.
So now Rachel, suddenly realizing what a great thing Ross did for her many many years ago, walks over to Ross and gives him a big ol' kiss. The audience applauds. There is joy and celebration across the world. People in LA riot, people in New York bite each other, people in San Francisco pretend they're too intellectual to watch TV, and people in Chicago are too damned cold to do anything.
Okay, back you go.