You play as Calvin who now lives with his niece Megan, and the goal is to corrupt her. It's a fairly modest game. While it's technically an RPG Maker MV game it plays nothing like a regular RPG as it is pretty much purely menu based with a point-and-click system.
In order for you to be able to install this game, you need to enable (allow) installation of applications from unknown sources (Unknown sources) in the phone settings. Usually this item is in the Security section