Light, a silver-and-black pony who has the ability to melt into shadows, shows up in Ponyville. After she is discovered by Twilight Sparkle, she is welcomed into the Ponyvillian community. Suddenly, cutie marks start disappearing. Everypony suspects the new pony, but she denies each and every accusation. However, every night, she disappears. What is this pony doing to Ponyville?