Steam Heart's is a 2D sci-fi vertically scrolling shoot-em-up with erotic content. The game takes place in the far future, on an alien planet dominated by seven elf-like or catgirl princesses that have fallen under the spell of a unique virus that has driven them mad and threatens to destroy the planet if all the rulers declare all-out war.

Only Blondia Varady (nicknamed "Blow") and his sister Fallandia have been spared from the disease and must pilot their fighter ships in order to penetrate deep into the domain of each princess, destroy their war machines, and administer the antidote. Fortunately for Blow, the antidote has been synthesized into his reproductive organ, so that means there is only one way to inoculate the girls....

The player controls the fighter ship through vertically scrolling stages, trying to destroy every ship that crosses its path and reach the end boss. The ship has a primary weapon that can be constantly upgraded by collecting power-ups, and a secondary slot for all sorts of collectible weapons that can be picked up in mid-air. The game features 2P-cooperative gameplay and cutscenes made of still images that explain the plot and depict the protagonist's encounters with each princess.

The game is set in a world where people have been taken over by a virus, altering their behavior. Two siblings, a male named Blow and a hermaphrodite Falla are immune to the virus and set off to fight it. The characters exchange dialog during gameplay to progress the story. The bosses at the end of each stage are mecha piloted by scantily clad women including catgirls. After each boss is defeated, Blow and Falla interact with the women. Blow ejaculates into the boss character to cure them from the virus.