Short version is; Twilight is ‘helping’ Shining Armor with his cadet helm by skedaddling off with the thing to polish it for drills, but failed to communicate that with her big brother. A vaudeville-esque chase through the castle ensues.
Celestia is amused. The bourgeois are decidedly not.
Cadance helps. Sort of.
No, I didn’t forget Twilight’s cutie mark, she’s very young here.