INSIDE is a puzzle adventure, where players assume the role of an lonely boy who swims, runs, and climbs through a desolate world to solve various puzzles and find out what happened. In the course of his travel, he also comes across a siren-like creature, who gives him equipment for breathing underwater.

The main storyline follows the unnamed boy through the world locations including a farm, a forest, and a fictional laboratory, where experiments on bodies are held.

Gameplay is divided into levels tied to each location. While completing them, the boy, controlled by players, has to swim, run, climb the obstacles, and use his supernatural ability to control bodies with his special helmet.