Mario and Sonic: Heroes Unite! is a CrossoverFan Fiction between the Sonic the Hedgehog and Super Mario Bros. franchises, created by Azurixx. It can be found here.The story starts as any Sonic game would go: Sonic's Arch-Enemy Dr. Eggman attacks Empire City and Sonic goes to stop him. A fairly common task for Sonic.That is, until a portal suddenly shows up and sucks Sonic, Tails, Amy and Eggman inside.When Sonic goes back into his senses, he finds himself in the Mushroom Kingdom. Now he has to find a way to find the rest of his friends who also got sucked up return to his world. Unbeknown to him, though, an evil force, bigger than Eggman and Bowser's powers combined, is threatening both of their worlds...The story's divided into three parts: "The Mario World Saga", which corresponds to chapters 1 through 46, "The Sonic World Saga", which corresponds to chapters 47 through 92 and the "Finale Saga", which corresponds to the remaining chapters.As of August 27th, 2013, all 100 chapters have been posted and the story is completed!
This Fan Fic contains examples of the following tropes:
April Fools' Day: According to the author's note from chapter 58, the author pulled out one of these when he pretended he was going to "leave" the story and stop writing. It was meant to be a harmless prank, but people reacted on the negative side.
Alas, Poor Villain: Bowser ends up being killed protecting Peach from an attack from Malevo.
He got better thanks to some 1-ups at the end of chapter 97.
There's an even BIGGER Bad, a dragon by the name of Malevo, the ruler of the Negative Dimension that Metal spent a long time trying to get into, and now that there's a pathway between Sonic's World and Mario's World, he's gonna level both of them to show the worlds what negative energy can really do! Believe it or not, this guy, NOT METAL, is the guy described as "bigger than Eggman and Bowser's powers combined"
Big Word Shout: In chapter 82, as Bowser is delivering the smackdown to Metal Sonic, Bowser Jr. suddenly yells for him to stop.
Book Ends: Of a sort. Chapter 2 has Mario saving Sonic from drowning in the lake in Princess Peach's castle. The second-to-last chapter of the Mario World Saga has Mario and Sonic staring at that same lake.
Criminal Amnesiac: Luigi suffers amnesia following the transfer to Sonic's world. Metal Sonic takes the opportunity to claim that Luigi is on his side and that the heroes are actually his enemies.
Disc One Final Boss: Mario and Sonic have defeated their Arch-Enemies in Bowser's castle—which was completely destroyed in the process. Sonic and his friends then successfully return to their world. The story's over now, right? Nope. Cue chapter 46 and the Sonic World Saga.
Another Disc One Final Boss happens in chapter 92. The heroes have destroyed Eggtropia after a fierce battle. Unbeknownst to them, Metal Sonic has reactivated and has left Eggman unconscious. Cue the Finale Saga.
Distressed Damsel: Bowser (of course) kidnaps Princess Peach in chapter 2. And again in chapter 40.
Doomsday Device: Eggman builds a robotic duplicate of Bowser, powered by a Chaos Emerald, which he codenames the Doomsday Machine and names the Giant Bowser Mecha.
Dudley Do-Right Stops to Help: Sure, he's busy finding out the whereabouts of Tails and Amy, but even Sonic couldn't resist helping out a friendly Koopa Troopa in need.
Earn Your Happy Ending: Sonic and Mario manage to beat Malevo and save their worlds, but now the guardian of the realms has shattered the link between the two worlds, leaving a sort of bitter-sweet ending.
Eleventh Hour Superpower: As per standard in the Sonic franchise, Super Sonic makes an appearance in Chapter 98. However, Mario also goes Super in the following chapter. As the author puts it, this gives a whole new meaning to the term "Super Mario Bros.".
Evil Is Not a Toy: Malevo plans to show both Sonic's World and Mario's World this, by taking over and conquering both worlds. Metal Sonic already learned this the hard way when he offered Malevo the Chaos Emeralds in exchange for his power, only to be torn apart.
Exact Words: "Tails said I couldn't destroy the robots. He didn't say they couldn't destroy each other."
Find Out Next Time: A narration summarizing the main events of the chapter is added after the "To Be Continued" happens and before the Author's Notes for the chapter are displayed. Later chapters finish this narration with "Tune in next time for the next chapter of Mario and Sonic: Heroes Unite!" or variations of this phrase.
Fission Mailed/Interface Spoiler: Chapter 43 seems to end on a downer: Mario and Sonic are trapped, Eggman pushes the Big Red Button on his Doomsday Machine, doing his trademark Evil Laugh... it seems like the chapter's going to end on a cliffhanger... but you haven't reached the end of the page yet! It certainly doesn't end like this!
Sorta happened when Luigi managed to steal the Red Chaos Emerald from our heroes.
Sonic receives a Power Star just before going back to his world. That same Power Star is used to power the Star Fighter the heroes use as transportation.
It Only Works Once: The characters from Sonic's world can only use ONE power-up from Mario's world. They can only use that one power-up after it's first use. A symbol of said power up will be marked on their right hand underneath their gloves. Tails gets the Fire Flower. Amy gets the Ice Flower. While Sonic gets the Rainbow Star.
No-Holds-Barred Beatdown: Bowser, filled with anger about losing his castle and never seeing his son again, completely trashes Metal Sonic in chapter 82.
The Reveal: Chapter 39 reveals that the shadow figure who was responsible for bringing Sonic to Mario's world in the first place was none other than Metal Sonic.
During Chapter 65 there's this:
Mario: Mamamia...Luigi, is that you?
Recurring Element: When ever a character gets some type of power-up, after a description it will say "[name of character] was now [name of form.]"
Will They or Won't They?: While Sonic and Amy are in Mario's world, hints that there might actually be a romantic interest between the two of them are seen. When they return home it appears to be gone, but they have a touching moment when the first fly in the Star Fighter during Chapter 64.