When Discord is reformed by Fluttershy, he returns to Canterlot along with Celestia to review his marefriend. Discord realizes that although Luna is happy to see him again, she is sad about what she did in the past.
Celestia realizes that Luna is very sad after Discord's petrification. Celestia, concerned for her sister, she is determined to discover the reason for Luna's sadness.
Following just after the events of "Make New Friends But Keep Discord" Princess Luna tries to join in the fun but ends up heading back to her room to resume her duties of the night. That is until Discord appears and offers her for a dance.
After being banished, Nightmare Moon finds she is able to talk with Discord, also sealed away. The two despise one another, but talking with a hated foe is better than endless monotony. Over time, the two fall in love, up until tragedy strikes.
Twilight has noticed something strange since becoming a princess, Luna and Discord hardly acknowledge each other's existence outside normal expected royal greetings. And Twilight is determined to find out why.