What Should the plot of my story be?

I am writing a story about three girls who live in a house for boarders that their foster father owns. They are looked after by their house keeper and her daughter, thier father who found them all on seperate expoditions around the world has now left the family for another one and is lost on an island (he has nothing to do with the story line so ignore him)because he left the family is now poor but happy, what should happen to them to make it interesting?