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. Then, Billy and Grim each tell disparate stories on how they became best friends originally. Billy's fantastic story is about he 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). In the end, Mandy quickly tells the real story, and footage from the first episode is seen, before the four are swallowed whole by the serpent.

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S07E01 - Everything Breaks