Twilight Sparkle finds an ancient star chart which shows the location of another world, Earth, as well as a powerful teleportation spell capable of interstellar travel. Princess Celestia allows Twilight to go to the other world for exploration. However, due to the Changeling invasion during the royal wedding, the negative energies it produced also frees Discord from his stone prison, who also decides that Earth should be his new playground. Meanwhile the Autobots continue to defend Earth from Megatron, and his Decepticons; ignorant that a new player is about to release chaos upon the world. And Megatron is not about to let another would-be conqueror steal his prize. Equestria also has it's problems when Queen Chrysalis returns to Canterlot, bent on usurping Celestia and Luna.
Twilight Sparkle, and Optimus Prime must save two worlds from three powerful villains.

(This takes place at some point during the 20 year time frame between The Transformers G1 series Season 2 and Transformers: The Movie; and 1 week after the Royal Wedding in Canterlot.)