After you defeat all four bosses in planet Zebes, you can reach Tourian and battle the final boss.

Tourian

After you have gone down the elevator in the statue room, you can go through the door on the right to use the save room. This is the last save room before the point of no return, so be sure to save here and not at any later save points! Exit the save room and go through the door on the left.

Metroids will attack you here. You can freeze them, but you will need to use missiles to kill them. Metroids can latch onto you and start draining your energy. You can use a bomb to try to break free, or three Power Bombs to kill it.

The turrets in this area will shoot Rinka (ring-shaped beams of energy) at you. You can destroy these with uncharged beam shots and the Screw Attack.

Go to the left through this first room with the Metroids. You will need to kill all of the Metroids in the room to unlock the door on the left. As mentioned above, you have to use missiles to kill them. After they are gone, go through the door on the left.

Kill the two Metroids in this room as you go downward, then go through the door at the bottom.

Go to the right through this room, dodging or destroying the Rinka beams and killing all of the Metroids to unlock the door at the right. Go through.

Go downward through this room, killing the Metroids to unlock the door at the bottom. Go through.

There are Sidehoppers in this room that you can kill with Super Missiles. Go through the door on the left.

If you touch the Torizo in this room, it crumbles to dust. The door on the left is the point of no return before the final boss! If you are ready, go through the door on the left.

There are several enemies in this room that crumble to dust if you touch them. Go to the left side and you will see a blue Sidehopper. After a moment, there will be a cutscene. Try to go to the left and watch what happens. Eventually, you will be able to go to the left again. Don't use your reserve tank energy; you won't encounter enemies up ahead. Go to the left and shoot through the webs on the left to open the path. Then go to the left to reach the next room.

There are no enemies here, so shoot downward and go through the door on the left to reach the recharge room, where you can recharge energy and missiles. Then exit the recharge room and shoot the door on the right with five missiles, then go through.

There are no enemies in this room, but there is a Gadora on the door on the right side. Shoot it with missiles when it opens its eye, then go through the door.

Don't Save the Game

Go down through this room while dodging the Rinka beams that the turrets shoot at you. It is NOT—I repeat—NOT recommended to save in the save room here! You have already passed the point of no return, so if you want to get 100% completion and you missed anything, you won't be able to go back if you save the game here! Go to the bottom of the room. Shoot the door with five missiles and go through.

Dodge Rinka beams and bullets. There is a glass tube in the way. It is vulnerable to missiles, and will regain health over time. Shoot missiles at the glass tube to destroy it, then go to the left. This time, you find a pair of glass tubes. You only need to destroy one to destroy the other. After that, go to the left and destroy the next tube, then there is another pair of tubes to the left. After that, you finally reach the final boss: Mother Brain.

Mother Brain

Phase 1

You will have to dodge Rinka beams and bullets throughout the battle. Shoot missiles at the glass tank until it breaks open, then shoot Mother Brain with missiles until her housing explodes. After a moment, she will rise up from the ground.

Phase 2

Shoot missiles and charged shots at Mother Brain's brain. She will try to walk forward, but you can push her back by shooting her brain.

She will sometimes drop a bomb, which will do a spreading explosion after a moment. She also will sometimes shoot a ring-shaped beam at you, and sometimes a red laser.

After her health is partly reduced, she will start shooting an exploding red beam at you.

Eventually, she will start charging an attack, then will hit you with a rainbow-colored beam. You will lose many missiles, and will lose all super missiles and power bombs. You can keep shooting her with charged shots, and she will start charging another attack. After more of Mother Brain's attacks, there will be a cutscene.

Phase 3

You will start glowing with rainbow colors. You can now shoot the Hyper Beam. Mother Brain will attack rapidly. Shoot her brain with the Hyper Beam while dodging her attacks. Eventually, she will turn red and start exploding. After a cutscene, a countdown timer will begin.

Escape

Go through the opening in the left wall. Shoot the Hyper Beam at the gates in the way to destroy them. Shoot the door in the floor and go through. Go to the right side of the room and shoot downward to kill the Space Pirate in the way and open the door, then go through the door. Jump onto the ledge where the Space Pirate is standing, kill it with the Hyper Beam, and run to the right. At the end, jump up onto the ledge and run to the left, then jump up through the gap and run to the right. Go through the door on the right.

Go up to the top of the area, then run to the right, go down, then go to the left, and go down to the bottom of the area. Go slightly to the right, then go straight up until you can't go farther up, then go to the right and go through the door.

Shoot to the right to break through the wall and reach the tall area. The acid will start to rise. Go up to the top of the area (you might find it easiest to just land on each platform instead of trying to Space Jump) and go through the door there.

Go to the top of this area and go to the right. You can optionally roll into the tunnel and go through the door at the bottom, then go through the door on the right, where you will find the Etecoons and Dachora. You can save them by shooting the bottom-right wall of their room. Wait for them to go off screen, then go to the left through the previous rooms, and go up the right side of the room and roll to the right through the tunnel. Go through the door on the right. From there, just run to the right, jump onto the top of your ship, and press down to enter the ship. Congratulations! You won! Enjoy the ending!