The player pilots a lone fighter ship through a quadrant of the galaxy, initially blasting away at drifting planetoids to "mine" Sinisite Crystals from them, which, when harvested, create Sinibombs. Sinibombs are the only weapon capable of damaging Sinistar. The fighter's bomb bay can hold 20 Sinibombs.
The player creates Sinibombs by shooting at the drifting planetoids and catching the crystals released. Each shot makes a planetoid shake, and if too many shots are fired at a planetoid at too fast of a firing rate, it will cause the planetoid to explode and no crystals will be released. At the same time, a planetoid must shake a certain amount to actually release any crystals. It normally takes between 3 to 6 shots (depending on the size of the planetoid) to release between 1 and 10 crystals. Additional crystals can also be obtained from the same planetoid, as long as the number of shots and firing rate do not cause it to explode.
At the same time that the player is trying to get crystals, the "worker" ships are also trying to take the crystals. They use these crystals to create Sinistar, the skeletal-looking boss who is trying to destroy the player. At the same time that all of this is going on, other ships (called "warrior ships") are trying to shoot the player's ship, and the warriors can also be seen shooting at planetoids to mine crystals when they aren't trying to attack the player or guard the Sinistar.
If the player's ship is destroyed before the Sinistar is formed, the game shows how far along the worker ships are in building the Sinistar. The workers must harvest 20 crystals before the Sinistar is completely formed. Once it is completely formed, a digitized voice says "Beware, I live". While he is trying to collide with the player's ship in order to eat it, he says further things. His seven voice clips are "Beware, I live!", "I am Sinistar!", "Run! Run! Run!", "Beware, coward!", "I hunger!", "Run, coward!", and a loud roar. If the Sinistar succeeds in colliding with the player's ship, the ship spins out of control and then the Sinistar eats the ship, which also causes the ship to explode.
