In a multicultural city of the future, a state of emergency must be declared as suddenly - out of nowhere - portals to another dimension appear. Dangerous creatures attack the inhabitants and contaminate the surroundings. The traditional police forces are overwhelmed. A special unit called Neuron is established. You can choose between two playable characters, a man and a woman. The character you do not select appears in the game as your younger twin brother or sister and also works for Neuron. To resist the alien creatures, humanity develops a humanoid weapon: Legion. Legion accompanies you in your investigations, assists you, and fights by your side. Through synergistic collaboration with Legion, you develop numerous combat styles. For example, you can simultaneously attack the same enemy or take on different opponents. You can also send Legion on the offensive while keeping yourself in the background and supporting the fight with items. Together, you fight and explore.