Parasite Eve II is a single-player survival horror role-playing game that blends action-oriented combat with exploration and resource management. It is set three years after the events of Parasite Eve and follows Aya Brea, a former NYPD officer who now works for the Mitochondrion Investigation and Suppression Team (MIST), a division of the FBI dedicated to eliminating creatures created by mutated mitochondria.

The story begins in Los Angeles in the year 2000, when a violent outbreak of Neo-Mitochondrion Creatures (NMCs) occurs at Akropolis Tower. Aya is dispatched to contain the incident and soon discovers evidence of an underground organization experimenting with mitochondrial mutations. Her investigation spans deserts, research facilities, and abandoned towns, ultimately revealing a conspiracy that threatens the future of humanity.

Gameplay departs from the Active Time Battle system of the first game and adopts mechanics closer to traditional survival horror. Players explore pre-rendered backgrounds with tank-style movement, while enemies appear in real time rather than through random encounters. Combat uses a targeting system where Aya locks onto enemies, moves to evade, and attacks with firearms, explosives, or melee strikes. Defeating monsters earns experience points, used to unlock and upgrade Parasite Energy abilities, and bounty points, a currency for purchasing weapons, armor, and items.

Parasite Energy is divided into four elemental branches: fire, water, wind, and earth. Each offers powers ranging from healing and status recovery to offensive attacks like combustion or necrosis. Unlike in the first game, these abilities consume mana rather than turn-based actions, allowing combat to flow more like a shooter while still rewarding tactical choices. Weapon customization is limited, but equipment can hold attachments such as grenades, recovery items, or armor add-ons that grant passive benefits.

Exploration and inventory management are central to survival. Ammunition, healing supplies, and tools are scarce, requiring careful use of limited storage and strategic saving of resources. Checkpoints allow Aya to purchase supplies with bounty points, and players are encouraged to replay areas to farm resources or strengthen their abilities before facing major threats.