A mercenary teams up with a prince to rescue a princess who was kidnapped by her father who is an evil emperor.

I'm done.

- Two childhood friends, including a mercenary and a prince falls in love with each other in space until they decide to stop an evil emperor from having the town being invaded by aliens

• a mercenary and a prince.. who is your protagonist?
• 'falling in love' should not be confused as the inciting incident.
  It doesn't bring the story into action. Think of a specific event which makes her go against the emperor
  OR think what's at stake. She's in outer space, what does she have to lose with this invasion?

I revised it.

• Since the objective goal belongs to the emperor's daughter,
  hence this should be written with her as the protagonist
• To invade pertains to intrusive entrance; one must be unearthly, to invade earth.
  
• What's standing in her way seems missing. What prevents her from stopping her father?
• Is the action 'teams up to stop her father from invading earth... set in action by when a prince falls
  for an emperor's daughter or the rivalry between these families..? if not, these can be dropped out

Good luck