Twilight and her friends decide to skip the Summer Sun Celebration, and go camping instead. It's only for a couple of days. Nopony will even realize they're gone.

What they find when they return, however, they find that the whole town is completely deserted.
The apples on the apple trees are rotten.
Twilight's precious library has been burnt to the ground.
Sweet Apple Acres is in a worse state than ever.
The animals in Fluttershy's cottage have starved.
Celestia has disappeared.

What they will discover will completely change their lives forever.

Based on John Marsden's "Tomorrow, When the War Began" series.