A Pup Named Scooby-Doo (1988)
S1E2 The Sludge Monster from the Earth's Core
Episode Overview
At the Coolsville bank, a robbery takes place, and the janitor claims he spotted a monster in the vault. However, no one takes him seriously and he ends up being accused of the crime. His story gains credibility when Scooby-Doo discovers the monster hiding in his own doghouse, confirming the janitor's account.
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S1