The Grim Adventures of Billy and Mandy - Season 6 - Eps 24: Billy & Mandy Begins
As Billy, Mandy, Grim, and Irwin hang from a rope about to be eaten by a giant sea serpent, Irwin wonders how they came into that trouble. Billy and Grim each tell separate stories on how they originally became best friends forever and ever. Billy's fantastic story is about him and Mandy being space patrol officers who collect the Grim Reaper's bones, cloak, and skull to summon him; Grim tells a story on how the evil Mandy and his pet servant Billy managed to capture him inside a containment unit (parodying the Ghostbusters' ghost traps). Mandy quickly tells the real story, and footage from the first episode is seen, before the four are swallowed whole by the serpent.
About The Grim Adventures of Billy and Mandy

Title: The Grim Adventures of Billy and Mandy
First Air Date: 2001-08-24
Last Air Date: 2007-11-09
Status: Ended
Rating: 8.045/10 (from 958 votes)
Language: EN
Seasons: 8
Total Episodes: 184
Network: Cartoon Network
Genres: Animation, Comedy, Kids
Production Companies: Cartoon Network Studios
Synopsis
The exploits of the Grim Reaper, who has been forced into being the best friend of two children. A spin-off of the show Grim & Evil.
Cast

Greg Eagles
Grim (voice)

Richard Steven Horvitz
Billy (voice)

Grey DeLisle
Mandy (voice)

Vanessa Marshall
Irwin (voice)