Trigun: - Badlands Rumble Episode 1

: Vash is captured by the Dodongo brothers , who seek revenge for a 270-year sentence they received because of him.

If you meant the first episode of one of the related TV series, here are the details: Trigun: Badlands Rumble Episode 1

: Vash the Stampede intervenes, not to stop the robbery, but to save Gasback’s life during a police ambush. : Vash is captured by the Dodongo brothers

: A notorious robber named Gasback is carrying out a major bank heist. not to stop the robbery

There is a specific manga one-shot titled (sometimes referred to as the movie's "Episode 1" in fan circles) that provides additional backstory: