Super Mario Sunshine is very similar to its predecessor, Super Mario 64, in that the gameplay is open world, with linear story milestones. This walkthrough is split up to lead you through the linear milestones, but also provide all of the details necessary for clearing the open world areas.
The Isle Delfino has several regions to explore. There is a central area, Delfino Plaza, and several "adventure" or "episode" areas which branch off from it. Each region is isolated from the rest, accessible only through specific portals, tunnels, etc. These areas are where the more traditional gameplay takes place. The game's goal is to reach the final linear milestone, but to do so you have to collect a required amount of Shine Sprites as you go along. The walkthrough shows how to gather every Shine Sprite, but you do not need to collect every single one in the game to complete the game.
The story begins with an introduction video of Mario, Princess Peach, Toadsworth, and some other toads making their way to the sunny beaches of Isle Delfino for vacation. As they arrive, they find the island in crisis. It has been covered with graffiti by a mysterious mustached miscreant and as a result the island's energy source, the Shine Sprites, have been scattered all around. It's now down to Mario to help the islanders return to their normal lives
After the intro movie, Mario and the crew have assemble outside of the plane at Delfino Airstrip, but something's not right...