In The Swarm, players navigate an alien-infested Red Square in Moscow, shown under the effects of a nuclear catastrophe. The game features a unique adaptive combat system, where players awaken in an alien laboratory and will choose throughout the game to either enhance their performance with alien DNA that increases attributes like stamina and health, or retain their humanity - and standard abilities - to fight the invading forces. While searching for an antidote to cure their allies against the alien mutation, players will solve DNA mini-games and puzzles for the option to absorb new abilities and become more adept at fighting tougher enemies. Players can then build and change alien DNA chains to improve different abilities like harvested energy from the bodies of defeated enemies, increased regeneration rate, enhanced vision or develop physical manifestations with access to energy weapons and sharp claws. Players can use a variety of weapons-including firearms, clubs, plasma projectors and grenades-for melee or ranged combat against enemies.