SPOILERS BEGIN HERE
Somewhere in the Star World, there is a city, that is of most beauty. It is abandoned, and contains a great treasure. A treasure that can do almost anything in its power! This city is filled with water adjustment and many, many traps. The water has remained on the ground for many milleniums. This city is very dark, but the buildings reach into outer space! There is supposedly a secret cannon, that will launch someone past a wall of Cosmesh, and to the treasure. Only one will reach this place, to come all the way up, and find the treasure, before it falls into the wrong hands.
In the Star World, Stario runs around excited, that he wants to bring Mario and friends over for something really cool. He runs outside, and into where he can trigger a switch to find mario without transforming himself.
In Mario's world, Mario goes for a little swim, and then runs into the castle. He goes to Bob-omb Battlefield and challenges Koopa the Quick as he Moon Jumps to the goal. He also slides around on all of the slides in the castle, Tall Tall Mountain and Cool Cool Mountain. To him, seemingly nothing is wrong.
Luigi decides to run around too, on an invisible shell nonetheless. He gets stuck, crashes a few times, but overall, has a good time.
Luigi meets MarioMario54321, who says he's waiting for someone, who turns out to be Cosmic Ball Kirby. A fan asks if there's anything, and Cosmic says it's a letter from the Star World, which reads
"Dear Mario and Friends,
We have made the most amazing discovery ever! This discovery is the greatest event to happen ever since you defeeated Mastar twice! You must come to the Star World right away! I will give you the details there.
Everyone gets very excited about this. They then run into the pipe outside to reach the Star World.
Everyone rushes in and sees the Star World again. Mario sees Stario, who explains that they've discovered a giant utopia that could change things in the Star World. The city is very valuable and the heroes need the Cosmic Crystals(which are locked away), 15 Light Crystals, 5 Star Crystals, 3 Moon Crystals and the key to reach the city and claim the treasure.
Everyone gets pumped up about getting all the stuff to reach the city, especially with no Mastar to ruin things, especially with his past deeds. Luigi finally gets credited for the first adventure, as the heroes head out.
Back in the heroes's world, superwalgina100 ends up being a villain, as CandyCao7 remembers the horrible times he ended up with him helping the enemy with his brother and failing. Lemurboy07 also remembers when the Tyler-O's were getting hits because of the Cosmic Crystal. Super then speaks of avenging their brother, as what's next is unknown...
Someone in heaven hears of the Star World and wishes to return, as he thinks how. Meanwhile, Super, with a friend named Snowball, go ahead with a plan to freeze the Star World with the power of the Cosmic Crystals. Stario gives them to the Startoads, who say the 15 Light Crystals are in each of the 15 courses in the Star World. The 5 Star Crystals are in 5 of those 15, and the 3 Moon Crystals are special; 2 are in the courses while one is in the castle. Mario figures it out to be the slides, as the Startoad confirms that. Robot8664, a fan from the previous movie, wonders if his record on the slide in the castle can be beaten as a flashback shows. Mario and Luigi attempt the slide but both fail, as fans begin collecting Crystals that are needed. The castle slide record is eventually beaten as the first Moon Crystal is collected.
Eventually, 3 Light Crystals get collected, as Super and Snowball try to get in the clock as Candy and Bandy and Tyler and Dude tried in the first and second movies respectively. Snowball fails to get in, but Super knows the code and gets in.
Rocko however declines knowing that Super is foolish and only wants to use the Cosmic Crystals with Snowball to take over the Star World. Super asks where they are, as Rocko squashes her out of the clock. She goes after Snowball as Candy and Lemur tell her he went to the coldest region of the Star World to look for the Crystals.
The fans collect the 4th Light Crystal, as well as the 2nd Moon Crystal. Eventually, the 5th Light Crystal is also collected.
MarioMario54321 noticed Snowball going away, as a fan chases after him. The 6th and 7th Light Crystals get collected. A fan manages to find the first Star Crystal in the volcano of the Star World Lethal Lava Land. Also, the 8th Light Crystal gets collected, along with the 2nd Star Crystal.
MarioMario54321 talks with a friend, as the logics behind everyone having brothers and sisters is questioned and unknown. The 9th Light Crystal gets collected, as well as the 10th, as a fan tries to figure out where Snowball is. He finds Snowball, who has a Star Crystal, with disgust of not finding his goal, the Cosmic Crystals. Two characters from the previous movie return, as Snowball teams up with the fan to defeat them. They're defeated and Snowball hands the Star Crystal over, making that the heroes's third.
The 11th and 12th Light Crystals get collected, along with the last Moon Crystal, of which the latter break the boundaries of the top floor password allowance in the clock. Snowball decides to live in the Star World Snowman's Land, not wishing to help Super anymore, as the fan isn't bothered by it at all. Superwario comes back as Superwalgina gets happy her brother is back.
The 13th Light Crystal is collected, as is the 4th Star Crystal. The heroes only need 3 more crystals until they are done.
Rocko is angered that the password is broken and will not live to see it. He makes their progress harder by calling friends to speed up features in the clock. Mario rushes up, noticing the crazy clock. Rocko tells him he gave the clock a password to protect the Cosmic Crystals just like in the first adventure when Mastar took over. He tells Mario if he can be helped, he will tell the location of the Cosmic Crystals along with giving the Light Crystal over. Luigi and LuigiLuigi09876 both collect Power Stars, which causes the clock to run at normal speed. Mario recives the Light Crystal, as he reveals the Cosmic Crystals are in the Secret Aquarium in the castle. Eventually, the last Light Crystal is collected in the Rainbow Ride of the Star World. Luigi finds the Star Crystal in this area, thus ending their crystal search. However, Luigi notices something in the fireplace and collects it.
Mario takes it to show Stario, who is shocked they found 1 of the 5 pieces of something, which is the key to the lost city. Two of them are in the castle, as the other two are elsewhere. Star Temple from the second movie has one. The Cosmic Crystals haven't been obtained yet, but they use the cannon to get there. Mario retrieves the second piece by flying with an owl to a cage.
The third piece is past the Star World Bob-omb Battlefield, in the Cave of Sadness. Mario recalls the events from the first movie, and is annoyed about being here again. However, when he goes in, all he sees is happy faces, and is confused of the change of events since the last time, feeling the same happiness when he recieved the Cosmic Crystals before. He proceeds through and finds the piece, feeling happy.
As the heroes were puttiing the key together, someone came and stole the two pieces they already had, who was an old fiend of theirs. Mario runs to it and sure enough, it's Mastar. Mario defeats him and obtains the key piece. However, Mastar is still at large with the last piece. Mario finally gets that piece too, giving the heroes the full key.
Stario manages to find out the true name of the place: Cosmicalitopia. Luigi is confused, as Stario explains it's the city at the end of the Star World. Mario gets the Cosmic Crystals quickly, as they teleport right to the wall. They use the key and get through, to Cosmicalitopia.
They reach the outskirts of the city, to which they use a cannon to reach the full city. Mario explores this city, changing the water levels, until Mario finds a hole. Luigi launches himself at the hole, as MarioMario says they must claim it before it goes wrong, and wonders about LL09876. LL lowers the water level but a fan rises it again and discovers something that grabs Mario's attention. Mario finds the cannon and launches after Luigi.
MarioMario launches after him (as a blooper scene is made by mistake) and LL09876 as they go down after the treasure. The fans launch after as well.
They find the treasure's location, but it is sealed. Luigi, MM54321 and LL09876 help to find red crystals to find the treasure, as a fan helps too. Mario uses the Star, as Mastar comes and tells them the treasure is actually the soul of the god of the Star World, whose name is Cosmicalitarus. Mastar did what he did to stop it from getting worse, but it's too late. Mario fights Cosmicalitarus, and wins however. It takes a very long, and difficult time. During the fight, Fleskhjerta and Megaman765 cheer Mario on. Cosmicalitarus finally sees how perfect the Star World is, as he fades, to rest alone.
Mario flies back through the outskirts to the Star World Castle. He reaches the pipe and goes home, to where he collects purple coins. He thanks everyone who helped, as Luigi reveals Mastar isn't evil anymore and just chills in Cosmicalitopia. Everyone is happy for Mario, as he collects all of the 100 purple coins and collects a Star in front of the castle, ending the story.
SPOILERS END HERE
- Fleskhjerta (cameo)
- Megaman765 (cameo)
- Mastar (after reform)
- Mastar (before reform)
- The Cosmic Crystals III: The End of the Star World on the MM54321 Wiki