From the game box:
A divine balance is disturbed as darkness encroaches on the world, and four chosen youths are entrusted with the final vestiges of hope.
User summary:
Final Fantasy III was the third and last FF installment for the NES. It was originally never released in the US. FFIII features a job-changing system, where the players could revert the player characters into warriors, mages, thieves etc, mid-game. However, the characters needed to stay in one class during a certain number of battle before being able to change again. The dark atmosphere from FFII is replaced with a light fantasy story regarding the restoration of nature's balance through the finding of the four crystals.
Not to be confused with the SNES Final Fantasy III, which is the US title of the game Final Fantasy VI
FFII and FFIII were never released in the US, so when FFIV was to get a US release, it was renamed to Final Fantasy II. FFV was also never released in the US, and consequently, when FFVI was released in the US, it was renamed to Final Fantasy III. When the FF series turned to the Playstation and FFVII was released, the US version got the same numbering and after that, all US releases of remakes of the earlier games have retained their original numbering.