Returning to Equestria from the portal to Canterlot High, Twilight returns with a video camera and a player to play the recordings. The Cutie Mark Crusaders are instantly enthralled with this device and want to take it for a test run, to see if they can get their cutie marks in Video Recording.
After a few days, however, things go terribly wrong. Ponies begin vanishing without a trace. No one makes the connection, until the final victims of the mysterious vanishing are the C.M.C. themselves. Distraught, Rainbow Dash, Applejack and Rarity struggle to find the lost fillies.
Meanwhile, the C.M.C . struggle with their own demons, both around, and inside themselves. Having run away to spare other ponies the punishment of their curse, they cling to one another in hopes of a salvation they know won't come. Shadows and figures that weren't there before begin appearing on the videos and the C.M.C. grow increasingly paranoid as they all feel like something is watching them.

Note: If I had to place this in the series, I would say it's obviously Pre-Raiders of the Lost Mark, but Post-Equestria Games. However, this takes place outside the usual timeline, so nothing is set in stone.