Although it isn't in the title, The 3rd Birthday is actually the sequel to one of Square-Enix's more obscure franchises, Parasite Eve. Released back in the PlayStation era, the first game was an unusual RPG. It was set in modern-day New York City and starred police officer Aya Brea as she battled a series of mysterious genetic mutations. It was as much a horror game as it was an RPG, and its odd blend of semi-realistic science and bizarre body horror made it quite memorable. The sequel, Parasite Eve II, was more heavily inspired by Resident Evil, although it maintained the franchise's focus on genetic horror and still had plenty of RPG elements. The new game, The 3rd Birthday, is oddly free of the Parasite Eve title, though much is different about The 3rd Birthday when compared to the previous games in the series. Fans hoping for a return to form may be disappointed, but The 3rd Birthday has some surprisingly interesting gameplay that may help longtime fans overlook the deviation.

The 3rd Birthday is set a few years after the previous Parasite Eve title. The Twisted, a mysterious new breed of creatures, has appeared through portals in space and brutally murdered humans.  They are seemingly unstoppable and have devastated New York City in a short period of time. Humanity's only hope against the Twisted is the hero of the previous games, Aya. Due to certain mysterious circumstances, though, Aya isn't quite herself and has been separated from loved ones. Using her new power of Overdive, Aya is forced to confront the Twisted to figure out what happened to her and the origin of the strange monsters. Revealing anything more would risk spoiling the plot, but dedicated fans can expect a number of cameos from characters from the first two games.

Since the first Parasite Eve was an RPG with action elements and the second game was like Resident Evil with RPG elements, it should come as no surprise that the third game deviates more from the original than the sequel. On the surface, it resembles a fairly traditional third-person shooter. Gameplay consists of moving around the stages and shooting foes, while occasionally taking cover behind walls to chuck grenades or recover health.

The 3rd Birthday still has a fair chunk of RPG elements and doesn't really play like a traditional shooter.  There is little manual targeting, so unless you're using a sniper rifle or grenade launcher, you lock on to enemies instead. Cover exists, but almost all cover is destructible, so you'll want to learn how to dodge because it's dangerous to spend too much time hiding. Aya has a sidestep/roll move to weave between enemy attacks. If you master this dodge, you can keep fighting without resorting to cover too often.  Likewise, by default, health regeneration is pretty slow and requires you to be completely still to activate. You can heal during quiet moments, but it doesn't work out very well in combat.  Most shooters are about targeting the enemy accurately, but The 3rd Birthday is more about avoiding attacks while dealing damage.

Perhaps the most unusual aspect of combat is the Overdive system. Aya isn't technically on the battlefield; she's projecting herself into the bodies of soldiers, taking them over and granting them her powers. At any time, as long as another friendly soldier is nearby, Aya can switch the Overdive character. This lets you switch tactics on the fly to compensate for new reinforcements or sudden enemy ambushes. If you're having trouble with a heavily armored foe, you can Overdive into a sniper who is far overhead and pick off the foe from a distance. Likewise, if your current body is about to die, you can switch to a new one to refill your health. You can also take advantage of soldier positioning to activate a Crossfire attack. If friendly soldiers are surrounding a foe, you can order them to all attack the foe at once for drastically increased damage.  However, there are some downsides. Treating borrowed bodies poorly means that you'll earn a bad score at the end of the stage. To finish levels with a high ranking, you have to defeat all the foes while causing as few friendly casualties as possible.

Overdive also works on enemies. By repeatedly attacking a foe, you can gradually stun them, and then you'll have a brief window during which Aya can Overdive into a foe. This does not allow her to take over the enemy's body, but it does massive damage to the foe — far more than any of her other attacks. In addition, Overdiving into foes gives you a chance to get an Over-energy Chip from them, and that's crucial for leveling up. Overdiving into enemies comes at a cost, though. As soon as the Overdive is finished, Aya is stunned for a few seconds. Even after she recovers, she has a temporary debuff that causes her to take more damage from enemy attacks.  It's a risky proposition, it can be a major payoff if you do it correctly. The damage done by an Overdive can far outweigh multiple clips from your regular guns, and the earned chips can make Aya more powerful.

In addition to Overdive, Aya also has another powerful ability: Liberation. As you fight enemies, your Liberation bar fills up, and once it's full, you can activate it to temporarily go into a super-powered state. In Liberation mode, Aya is functionally invincible, automatically dodging attacks and gaining the ability to warp around the battlefield at high speeds. She begins dual-wielding handguns that do amazing amounts of damage, and her Overdive ability does substantially more damage. Going into Liberation mode can also save you from dying or being grabbed by an enemy. The only catch is that Liberation charges rather slowly, so if you use it in an easy fight, it isn't as effective as if you'd saved it for a tough enemy.

As you'd expect from a Parasite Eve game, Aya has a wide selection of weapons that she can equip. Weapons come in a variety of classes: Assault Rifle, Handgun, Launchers, Shotgun, Sniper Rifle and Special. In addition to that, each weapon is divided into various subclasses that show its specialty. For example, you can get a regular handgun that is balanced in most ways. You can also get a Magnum revolver that is slower than the regular handgun but does significantly more damage. Also noteworthy are Impact weapons, which are almost universally weaker than other weapons but have a higher chance of stunning enemies. By using an impact weapon, you can Overdive into enemies with greater regularity, but the weapons are also less effective at killing enemies. It can be a bit of a drawback if you'd rather kill the enemy without risking the downsides of Overdiving.

By default, Aya can always access her personal handgun. This weapon isn't very powerful but has infinite ammo and can be beneficial in some fights. Aya gains new weapons by Overdiving into the various soldiers around the battlefield. Once she Overdives into a character, she has access to the weapon he is using. This allows you to switch weapons on the fly by finding soldiers who are using different guns.  Occasionally, you'll find characters with special weapons like a Gatling gun or satellite cannon. If Aya dives into a civilian or other unarmed character, she'll only have her handgun. Aside from her handgun and the soldier's weapon, Aya can also bring up to two other weapons with her. Once she has used a weapon once, she can purchase it from the store after the mission — with the exception of a few special weapons, like the aforementioned Gatling gun. She can upgrade weapons and bring them with her into Overdived bodies. These weapons have their own sources of ammo, which won't replenish if you change bodies, so you need to find ammo sources located around the stages.

An odd element of The 3rd Birthday is that you can also customize your DNA. As mentioned above, you can earn Over-Energy Chips from Overdiving into friends and foes.  Each chip is made up of various bits of DNA, which can be slotted into a 3x3 grid that represents Aya's own DNA. Each chip has various abilities, such as improved healing or defensive abilities, increased effectiveness of Liberation mode, or a random chance of projecting a barrier or firing an energy shot from your handgun. Fitting the most abilities into your DNA grid involves some thought because once a chip has been inserted, it can't be removed, but you can overwrite it with another chip.  When overwriting chips, various things can happen. The level of the ability may rise or fall, the ability may be overwritten by the ability on the chip, or the ability may mutate into a new ability. You can re-roll the outcome of fusing chips together, not unlike Demon Fusion in the Shin Megami Tensei games, but getting optimal outcomes this way is time-consuming at best.

 An easier way to raise the level of your chips is by linking them together. By default, a chip's effective level is only as high as the highest level of that ability on the grid, so if you have a healing level 5 and a healing level 3 on the same grid, your healing ability will only be level 5. If you place similar abilities next to each other on the grid, they will link, and their numbers will be combined, so if healing level 5 and healing level 3 are adjacent, you'll have healing level 8. This takes up more of your DNA grid, so you can level abilities much faster this way, but you'll have fewer overall abilities.  While you'll eventually want to have a grid of nine unique powers, it can be worthwhile to sacrifice some space for greater effectiveness.

The Twisted are a dangerous enemy. They come in many shapes and sizes, and few can be easily killed. If you don't invest in heavy defensive chips, the Twisted hit extremely hard and can kill your bodies in a few hits. Certain Twisted are nearly invincible, and some can even kill you in a single attack. Since you have multiple bodies, you often can survive fatal blows, but it still is a painful loss. Switching bodies properly allows you to figure out the best way to fight the Twisted. For example, one Twisted is a wall-crawling monster that can grab Aya and render her motionless while it prepares to carry her off to its nest. If you switch to another soldier, you can free your previous body before it does. There's another particularly nasty variant called the Stinker, which hovers in the air and sprays poison gas that makes it impossible to advance. Switching to a sniper lets you snipe the Stinkers, allowing other allies to move forward without dying. The boss Twisted are even more deadly, each having a unique ploy that requires you to use Overdive. One boss is a giant and heavily armored monster, with a weak point on its back. You have to constantly switch between various soldiers to disable it and then position troops on top of buildings to snipe at it. This task is made more difficult as the Twisted climbs up the side of a building and smashes your unfortunate soldiers.

The 3rd Birthday is an odd sequel. Although it certainly has a lot of callbacks to previous Parasite Eve games, the gameplay and tone are quite different. It may be difficult for longtime fans to get used to Aya's rather striking changes. Others shooters, such as Clive Barker's Jericho, have attempted a body-switching mechanic, but The 3rd Birthday doesn't play like any other shooter on the market. The odd mix of action and RPG elements makes it a rather distinctive blend that's more comparable to a Kingdom Hearts title than Gears of War. The end result is a surprisingly interesting action game with many unique gameplay mechanics. It may not be the sequel that Parasite Eve fans were hoping for, but The 3rd Birthday has the potential to be a great addition to any PSP owner's library.