This is a clip from "Hot Stuff", the 17th episode (159th overall) from the seventh season of ABC's monster hit 1974-84 sitcom "Happy Days". The episode originally aired on ABC on Tuesday, January 22, 1980.
When owner Alfred "Al' Delvecchio (Al Molinaro) leaves the local teen hangout Arnold's early, he leaves his teenage ward and fry cook Charles "Chachi" Arcola (Scott Baio) in charge. After Joanie Cunnngham (Erin Moran) finally (after several past rejections) accepts a date with Chachi, Chachi becomes so overwhelmed with joy that he puts his feelings for Joanie in the way of responsibility, leaving the kitchen grill on and throwing his apron on the grill when he leaves, causing Arnold's to go up in flames.
Arthur "Fonzie" Fonzarelli (Henry Winkler), Warren "Potsie" Weber (Anson Williams), and Ralph Malph (Donny Most) get trapped in the restroom; so Fonzie, using his motorcycle helmet, tries to break them out using his head, but gets knocked unconscious. Luckily, Richie Cunningham (Ron Howard), Joanie's older brother, and his girlfriend and Lori Beth Allen (Lynda Goodfriend) are nearby making out in Richie's car when Lori Beth hears Potsie and Ralph's desperate cries for help.
Realizing what's happening, Richie and Lori Beth quickly rescue Ralph, Potsie, and Fonzie by tying a cord from their car to the restroom window's grate, breaking it away and bailing the guys out. Regardless, Arnold's unfortunately burns down. Later, as the gang surveys the damage, Chachi admits his irresponsibility that caused the disaster, which infuriates Fonzie. In the end, however, Fonzie offers his life savings to Al, volunteering to become Al's business partner as they set out to rebuild a more modern (for the Sixties) Arnold's.
This storyline was part of a decision by "Happy Days" creator/executive producer Garry K. Marshall and co-producer Lowell Ganz that the old Fifties-style Arnold's was no longer appealing enough to teen audiences; it was burned down, so it could be replaced with a more modern Sixties-style restaurant, set to coincide with the turn of the decade from the Fifties to the Sixties (just as in real time, the turn of the decade from the Seventies to the Eighties).