Pinkie Pie gains the ability to see dreams, and when the Cake twins start having bad dreams at night, waking up crying and sobbing, she takes the matter into her own hooves to keep them safe from the nightmares that plague them.
When she finds out that she can do this because of a dream parasite that's latched onto her, Pinkie needs help, and contacts the Princess of Dreams. Somepony cursed her with this parasite, but she has to stop it before she can get to the culprit, and stopping it reveals Pinkie has in her more than she thought.