First contact was in 1978: space invaders came to us with attack ships, hordes of them. The fighting was fierce; if it were not for the capabilities of a then secret, experimental Tank, Earth would have surely fallen. After first defeat (Space invaders-Taito 1978) aliens tried again twice to conquer the world without success (Space invaders part II, Taito 1980 and Return of the Invaders, Taito 1985). Recovering wreckage from the destroyed invaders and studying their technology, a group of scientists managed to build a new defensive unit: a tank able to fly and equipped with an electromagnetic shielding system (CG-30 prototype). In 1990 Taito offered the fourth space invasion with several innovations. Game starts with old invaders trying to land again on Earth; at this point CG-30 tank arrives to repel aliens. While fighting back the alien attack, we start a journey of 43 round in space towards enemy's huge mecha, to destroy it and then save the Solar System.

During our journey we'll have to fight either on Earth, using the tank, or on air/space, changing form into a flying ship. CG-30 is still a prototype, it isn't perfect: if is hit by an enemy bullet or it crashes into an alien, it has only one electromagnetic shield to protect itself, and, therefore, second bullet would be the final one. Its cannon isn't able to battle all those enemies: it can shoot a bullet per time. That's why you have to try to hit the ship flying at the top of the screen (under the score) from right to left or vice versa and it will drop a capsule, that, picked up, gives us temporary special powers, offensive or defensive. Any item duration is shown at the bottom middle of the screen, by a decreasing bar. Capsule (same as Arkanoid's) are marked with a letter and will give to our Tank, in a random order, either an electromagnetic shield or four barriers (like the first Space Invaders), that, hit by us, will move upwards and, shot by the aliens, will move downwards. These barriers aren't everlasting: after several alien crashes, rounds of bullets or anyway at the end of the round, they will explode.

The offensive powers given by the capsule are: powerup: shots in rapid sequence, Laser ray, Time (aliens will stop for a few seconds), Hyper Laser: more useful when we are nearly invaded, in fact an enormous ray just over our Tank/ship will destroy everything on its way. Then two powerful weapons: a bomb that explodes and destroys nearly everything at once and some missiles that will follow and then hit the aliens.
In the game there are two bonus stages: we have to shoot down some UFO (looking like the ones spotted all over the world...). Obviously their intents aren't peaceful: they want to kidnap some cows to make experiments on them. Earn points for each saved cow and for every hit UFO If you manage to destroy all UFO and save all cows you'll get an addiction bonus points.
Game doesn't end if aliens land (as in Space invaders); we will just lose one of the 3 available lives.

Certainly SSI91 is the best game of the series (with the parody "Space Invaders 95"): there are several scenario, lots of aliens and ships to destroy, sound effects, music, superlative graphic details. Definitely a good shooter, drawn from an already known story, made captivating by keeping game features of the predecessors.