The movie Scooby-Doo! The Mystery Begins is a prequel to the original Scooby-Doo TV series, and it tells the story of how Mystery Inc. was formed.
The movie begins with a scene showing a young Shaggy Rogers walking his dog, who he names Scooby-Doo. They become fast friends and inseparable as they enjoy spending time together.
Later, the movie introduces the other members of the gang: Fred Jones, Daphne Blake, and Velma Dinkley. They are all attending the same high school and are not friends yet, but they all share a love for solving mysteries.
The group is brought together when they are all sent to detention together. While in detention, they witness strange occurrences that lead them to investigate a ghost haunting their school.
With their different skills and personalities, they work together to solve the mystery and find that the ghost is actually a revenge scheme by the school's principal.
The movie ends with the gang officially forming Mystery Inc. and solving their first case together.