Twilight finds an old book tucked away in a hidden place, whilst rummaging through her collection. The book is full of information on dreams (lucid dreaming, shared dreaming, dream control, etc) and has detailed instructions on how it's all done. She tells the other ponies and it intrigues them. They all decide to enter a dream together to see what it's all about. It starts off as a fun and joyful experience, but it quickly turns more sinister as the girls come face to face with all their deepest fears and anxieties. Plagued with panic and confusion, they soon lose control and their happy dream becomes a terrible nightmare.

In order to restore order to their dream world, they must battle it out with the horrors of their own minds. Each of them face challenges that torment them and tear at their sanity. Alone, they crumble before the might of their darkest thoughts and worries. But together, they find strength and manage to fight through the trials that test them, and return to the waking world.

That's my suggestion anyway.