Rain is a 3D game with an emphasis on puzzle elements set in a city loosely inspired by mid-twentieth century Paris. In the introduction with watercolour visuals the protagonist is introduced, a young boy who meant to go to the circus, but is bedridden because of a fever. He hears a sound outside his window and sees the silhouette of a monster in the rain, chasing the silhouette of a girl. He goes outside, follows the ghostly creatures through a door of light and discovers that just like them he has become invisible. The game environment is drenched in continuous rain and only when standing in the rain does his silhouette become visible and can he be spotted. When in a dry environment or seeking shelter he is completely invisible. He decided to find the girl before the unknown monster does.
The game is set in a dark, 3D city environment with free movement and cinematic, fixed camera perspectives based on the boy's position. The boy can move around, run, jump, climb and interact with the environment. He cannot attack, so most of the game he needs to use the environment to distract enemies or find cover. He can push items around to create platforms to reach new parts of the environment. Many of the enemies, afflicted with the same invisibility except when in the rain, need to be lured away with distractions and sounds to reach passages and nearby items. In many cases it is also necessary to run away and hide.
The game consists of eight chapters. After the game is finished it can be played again to collect three memories per chapter. These provide additional pieces of the story and are presented in the game as orbs that come up from the ground, often hidden in non-obvious places. They only appear when the boy is near them and can be picked up by walking over them.