Complex sentence: The third in the "Scooby-Doo" live-action film series, the film is an origin story for Scooby and the Mystery Inc. gang, revealing how they met and the events of their very first case.
Split into simpler sentences: The third in the "Scooby-Doo" live-action film series, the film is a prequel to the first two live-action films. It's an origin story for Scooby and the Mystery Inc. gang, revealing how they met and the events of their very first case.