The Mane Six travel to Hope Hollow, where they find the town and all of its inhabitants devoid of color. This encourages them to solve the mystery and return the 'rainbow' to the Rainbow Festival.
When they arrive in Hope Hollow, Twilight Sparkle and her friends realize that their dream vacation is more like a nightmare. The town and all it's residents are devoid of color. So the Mane 6 take it upon themselves to solve the mystery behind this washed-out world in order to bring the 'rainbow' back to the Rainbow Festival and color back to the lives of their new friends.