They Breathe is a short action game where you control a frog. It dives down a deep lake and the game scrolls down vertically automatically. You need to survive by grabbing the few air bubbles that appear. If the frog is deprived of oxygen, its colour gradually turns from dark to light green until its dead. The main goal to complete the game is to reach the bottom of the lake and save the frogs that are trapped there. You can swim in any direction and the space bar is used to swim faster for a short while.
While going down you meet other frogs. You can save them by bumping them into air bubbles. When they turn dark green, they are saved and will swim up automatically. There are hazards as well. Various creatures chase you and there are different mechanics to get rid of them. Some of them need air as well and you can kill them by catching the air bubbles before they do. Others such as jellyfish need to be steered into the air bubbles, causing them to explode. Some creatures are able to trap you and then you need to reach an air bubble to free yourself. Variations of the creatures have a green glow and they release green bubbles that need to be avoided. There are also hazards in the environment that can hurt you. Even though the game takes place on a single screen, it is possible to briefly move outside of the borders.
After completing the game a code is provided to enter They Breathe: Beneath the Surface, an interactive gallery on the game's website.