Meta Knight wants to become the galaxy's greatest warrior, so in order to do so, he embarks on a long quest through Dream Land until he reaches the Halberd (Revenge of Meta Knight), and eventually flies into space and visits some of the other planets in the galaxy (Milky Way Wishes), with the intent of reviving the destroyed Nova.
Eventually, he finds the Seven Stars from the other planets, and uses them to resurrect and summon the mechanical comet. In return, Nova grants Meta Knight's wish to "fight the greatest warrior in the galaxy". In doing so, Nova summons the ancient warrior Galacta Knight, who was sealed away for fear of his overwhelming powers.
The newly-freed Galacta Knight then engages in what Meta Knight hoped for - a duel between the two. After a long battle, Meta Knight manages to defeat Galacta Knight, causing him to flash and explode, presumably destroying him.
The triumphant Meta Knight returns to Planet Popstar, safe in the knowledge that he is now the galaxy's greatest warrior.
Instead of battling through Nova and fighting Marx as the final boss, Meta Knight brings Nova back to life with the power of the seven stars surrounding Popstar (a depiction of Nova reassembling together again proves this) to duel the ancient Galacta Knight.
Meta Knight's moves
Meta Knight's moves are based off Kirby's Sword ability, with slightly different moves and different names. He flies in a similar manner to Wing Kirby.
Meta Knight slices with his sword. If he does this at full health, a ranged energy blade is fired forward, which is known as the "Knight Beam."
B (after Meta Chop)
Meta Knight makes a small uppercut with his sword.
B, B, B
Meta Knight performs multiple jabs with his sword.
Dash + B
Meta Knight moves forward and performs a lunge with his sword. Meta Knight's Lunging Slice is quicker, covers more distance, and is more powerful than Sword Kirby's Lunging Slice.
Meta Chop and Thrust
Midair + B
Meta Knight slashes forward with his sword.
Dash, Jump, B
Meta Knight spins around with his sword. Similar to Sword's Sword Spin move.
Down + B in mid-air
Meta Knight falls down, and points his sword down to the ground. Meta Knight's Down Thrust is stronger and quicker than Sword Kirby's Down Thrust.
Up + B
Meta Knight points his sword into the air. This is a move exclusive to Meta Knight (and Sword Knight), and cannot be used by Sword Kirby at all.
Meta Knight can accumulate Meta Points (by defeating enemies), and can store a total of 50 points. He can use up his points to use these moves:
Meta Knight's movement speed temporarily increases.
Meta Knight (and his Helper, if he has one) will have all of their health restored.
Meta Knight creates a few tornadoes that spread through the entire screen, thus damaging all foes on-screen. Does 190 damage points. Can damage Reactor directly.
The sub-game is mostly non-canon, being a rerun of the main sub-games of Kirby Super Star, but as Meta Knight. However, despite this, the ending of the sub-game, including the fight between Galacta Knight and Meta Knight, may be considered canon, as it is mentioned in the Japanese Kirby 20th Anniversary Encyclopedia.
Meta Knight is significantly less clumsy than Kirby is during cutscenes. For example, while Kirby fell into the pit on accident, Meta Knight simply flies in. Also, Kirby lands face-first sometimes after using a Warp Star, but Meta Knight slides instead.
During the credits reel, certain scenes are shown that do not appear in cutscenes. One such scene shows Meta Knight knocking Kirby off the Halberd, while another one shows Meta Knight facing off against Dyna Blade. The final scene in the reel shows Meta Knight's mask on the ground behind him, although he is facing away from the camera, obscuring his face. These were likely added to make Meta Knightmare Ultra more unique, or it's possible the images appeared in cutscenes that were scrapped during development.