Pinkie Pie was planning a dance party and she was trying to learn to dance and turns to Fluttershy to dance with her and she agrees.

When they finish practicing the dance for the second time, they end up as one.

When the girls look for Twilight to see what happened to them, she looks up a book and finds how to fuse and starts studying Fusion.

Twilight starts explaining everything to Celestia and Luna and the Princesses know everything about Fusion because the Royal Sisters fused themselves.