After a spell miscasts, the Mane 6, the Crusaders, the Princesses, and Derpy Hooves suddenly get stuck in the world of Warrior Cats. This story takes place after "The Last Hope."
When a Portal is discovered on the edge of the everfree forest, some ponies go to investigate. When they get there, they are sucked into the Portal which leads to a new world altogether- and the ponies are no longer ponies. They're warrior cats.
Crookedstar receives a prophecy that six brave young warriors will save the clans at their time of need. Little did he know that those six would also end up being four legged as well.