The 3rd Birthday is quite different from the original Parasite Eve. Aya Brea, the heroine with evolved mitochondria, is still the lead character and the game is set in New York. Future New York. 2012, to be specific.
The 3rd Birthday, however, is not a role playing game. Square Enix made a straight third person shooter with a body snatching system.
Hajime Tabata, Director, explained the overdive system during a behind closed doors press demo. Aya can take over the mind and body of nearby allies. When Aya dives into another character she becomes them, character model and all. Think Agent Smith from The Matrix. Each character on the battlefield fights on their own when Aya isn’t controlling them. Allies also have different weapons, so you may want to dive into someone armed with a shotgun if it’s effective against a particular type of Twisted, the monsters in The 3rd Birthday. Other times you’ll want to dive into an ally to save them. If a computer controlled character dies you won’t be able to “borrow” their artillery.
The overdive system can also be used to quickly jump between spaces. Aya can pass through walls and large gaps by scanning for allies and diving into them. In another demonstration, the overdive system was used to keep Aya alive. During a boss fight against a demented beetle with a razor horn, Aya had to use the overdive system to escape before the beetle trampled her.