With the Blueblood family under investigation, anypony capable of preforming a safety inspection is preoccupied. The next best qualified group just so happens to be the Bearers of the Elements of Harmony, according to Celestia. And so, the Mane 6 are sent off to fill in for the absent experts and evaluate the Dranorth Magical Science and Research Facilities.
But as with any story being worth told, and some that aren't, the trip turns into more than just an inquiry if everything is accident proof...