Saddled with nearly a billion bits of debt, the Flimflam brothers and Trixie have come to the conclusion that gainful employment is a sucker's game. With the Crystal Empire recently rediscovered after a thousand years, though, it is possible that another ancient city, lost for about as long, might be about to re-emerge. Of course, when it does, the Elements of Harmony will doubtless be dispatched - but if they could just find it first, they might be able to reap the rewards before they show up.
The chances of failure resulting in humiliation and/or death are high; the chances of musical numbers, equally so; but they have to try. The alternative - working - is just too terrible to comprehend.
(This is a sequel to Flim and Flam Save an Orphanage, but that story probably isn't required reading if you're good at picking things up from context. Just know that it's set early in Season 3 and that the events of Magic Duel did not and will not happen, and the rest should be easy to work out.)