Spike gets captured by an alien group called the Kraang and is brought to another dimension away from his home, he is rescued by 4 mutant turtles and is taken in by them and trained to stop them and return home
There is a secret held in the dungeons below Canterlot, deeper than the crystal caverns into the roots of the mountain. This secret is a creature imprisoned by the Princesses. It is a source of great knowledge, but may ultimately be a great enemy.