The story of FusionFall begins thousands of years in the past with the Planet Fusion, a world-conquering alien entity that absorbs the planets of its victims. Commandeered by Lord Fuse, each attack has given it greater gravity and power as it moves on to its next target. Things come to a head when Fuse turns his sights on the Earth, home of the Cartoon Network universe. The invader strategically sends out probes to the planet to gather information and start the assault. Quickly, his minions evolve and adapt to the Earth's environment, taking on distorted fusion versions of common life forms, and through the use of Terrafusers, they begin to terraform the planet.

The residences of the Cartoon Network universe, both heroes and villains, join forces to repel the invaders, but they are severely ill-equipped against Fuse and his minions. The battle is nearly lost and Fuse lays waste to a majority of the planet.

Academy opening narration

The opening to the new Academy training is narrated by Ben. See below for a transcript.

Pic (left to right): Mount Neverest, the Saturdays Airship, the Sector V Treehouse, the Powerpuffs flying past Waybig in the clouds.
Ben All in all, our world is a pretty amazing place. I mean, what's not to love?
Pic (left to right): Titan is standing in the back ground alongside the KND Treehouse, while Flapjack, Bloo, the Powerpuffs, Dexter, Computress, Jack, Zak, Fiskerton, Chowder riding Kimchi, Rex, Jake, Finn, Ben, and Ultimate Swampfire sidescroll in the foreground.
Ben Mutant toddlers, imaginary friends, giant Robots, child geniuses, evos, cryptids... that guy. Not to mention your occasional alien hero.
Pic (left to right): Mandark, the Ice King, Father, Argost, Mojo Jojo, and Fuzzy are in the foreground, while Stickybeard and the Toiletnator ride the Sweet Revenge above, and Hex and Demongo float above them. Also, Aku has a faint silhouette in the top right.
Ben [Commenting on the villain slidescroll] Sure we have our fair share of danger. The occasional evil mastermind or tyrannical monkey set on ruling the world...
A wide-shot Planet Fuse
Ben Still, none of us were prepared when Planet Fuse arrived, ready to consume our world.
(left to right): Fusion Bloo, Mac, Ed, Jack, Mandy, Frankie, and Dexter stand together in the foreground. In the background a Graveworm, Scribble spawns, a Toxic Spawn, a Creeper King, and a Scythe Simian head towards the screen over an infected landscape. Fusion Blossom and Bubbles soar overhead, while Planet Fuse looms in the distance.
Ben His creatures rained down, taking on familiar forms, and transforming the Earth itself.
DexterA slidescroll of Dexter, Computress, and Mandark display Nano Bloo, Aku, Fourarms, Numbuh Five, Grim, Ed, Mojo Jojo, Dee Dee, Dexter, and Humongousaur are shown scrolling in from left to right, respectively.
Ben But we learned we could harness their essence to create our greatest weapon against the invaders... NANOS!
Another slidescroll from left to right shows: Mandark, Computress, Dexter, Edd, Jack, a Mutant Eggplant, Buttercup, a Chicken from Outer Space, Corus, Manus, Fiskerton, Zak, Kevin, Wilt, Billy, Numbuh One, Numbuh Five, Finn, Rex, Mojo, Gwen, Ben, and Nanos of Rex, Alien X, Finn, Ed, Dee Dee, and Titan rushing towards the screen, ready for action.
Ben Let's face it. Saving the Earth is going to take all of us. Together!

Academy Opening

Fusion Fall - Beginning Intro

Fusion Fall - Beginning Intro

Academy Opening Video

Further reading