You see that's just a bad trailer. It spoils itself right out of the gate. Obviously Jack convinces Raggaro and the mantock pirates to wage war on King Mantrock. Asking that inherently implies where the plot is heading. I mean you can assume that the humans are freed because the subheading is 'Mermaid Saviors' also because an audience would probably not like a movie about people being slaves as a rule(e.g. all the movies ever about slaves becoming free).
That said, I would still watch this movie. Only if Willem Dafoe can play all the characters. We have the technology, the time is now! Make a movie with Willem Dafoe playing all the characters!