Does anyone know how I could write this?
Are they two Italian plumbers?Step by step.
First step - rescue the princess from the monster.
Second step - princess rescued, but now the kingdom is being invaded by an army of monsters. Rally the troops, man the battlements.
Third step - monsters defeated, but now the kingdom is in shambles, and a lot of people are saying 'we don't need any stinking royalty.'
I'd take some advice from DMing in D&D here.
Your best friends are "Yes, and" or "Yes, but."
You manage to clear out the mutant, giant rats in the tavern's basement, but you discover a hole leading into the town's sewers that the rats came from. You clear out the giant rats from the sewer, but in doing so you find the dusty, abandoned laboratory of an ancient necromancer in the course of doing so. You report the laboratory to the higher ups in the adventurer's guild, and suddenly you got dozens of knights from the capital descending on the town to take over the case. Etc and so on.
Yes, you learn what the true mission is (in all its appalling enormity) at the end of Act 2.Watch Star Wars, episode 4, A New Hope.
That evolves from a simple "rescue the princess" mission into "You have to save the galaxy by blowing up a deathstar and become a Jedi."
It's usually that the quest delivers some new information or problems that trigger the bigger story.