Finish the space level and get access to the command center.
Beat the boss of stage D.
Reach the checkpoint of stage E.
Beat the boss of stage E.
Beat the first boss.
Reach the checkpoint of stage A.
Beat the boss of stage A.
Reach the checkpoint of stage B.
Beat the boss of stage B.
Reach the checkpoint of stage C.
Beat the boss of stage C.
Reach the checkpoint of stage D.
