It is Diamond Tiara's 14th birthday and as a gift, she is going to be Princess Twilight's assistant for the Grand Equestria Pony Summit. While Diamond considers it to be lame at first, she soon finds that speaking on Twilight's behalf is more fun than she thought.
(Written for Skeeter The Lurker's Switcharoo Contest. This fic is essentially "Princess Spike", but with Diamond Tiara instead.)