After you defeat Ostroid the robot, you emerge in a maze-like area of the spaceship.

Get through the Maze

To the north, you can see the second magic cube, but you will have to go through a maze to get it.

Go east. When the path splits, go east, then at the next split go north, then when it splits again, go east and use the stairs.

Follow the straightforward path and use the stairs there.

Go east along the northern path, and use the stairs that you find there. From this point on, the path is straightforward, so keep going until you find the Magic Cube. Take it, and your life level will be fully increased, with a maximum capacity of 22 hearts. Your health will be filled to maximum. From now on, when you pick up a capsule, it will fill your health completely, and it isn't temporary.

Then there will be a cutscene, and you will proceed to Chapter 8: Final Battle.