First arc ideas

I want to start by saying im NOT a first time dm. I've been doing for the last 4 years.

Hi im designing my campaigns first major arc. And im stuck and dont know how to implement it fully.

The gist is, the party is on an island nation, Polindu. Several other kingdoms have bases on this nation. And one nation wants to take it all over, im calling them Vaalbaara. Also the queens son has come home from his study abroad trip. What rhe queen doesn't know is that her son is working with Vaalbaara to overthrow her so Polindu can be their vassal state. I know i want to to end it with the party fighting the son and the leader of the vaalbaarans, while the queen his hooked to some machine that siphon her magical energy.

The arc starts with them meeting thw queen after helping one of the islands with a bandit problem. The only problem is I dont know how to go from the start to the finish.