Legend says --

There was a white dragon living in River Long En, preventing floods, falling rain. People worshipped him as the River Lord. By coincidence, he found out the annual festival celebrating the river god had been abandoned for years, angered, the River Lord Huan Yi swore that the Long En town will never see another drop of rain.

What appeared in the pond of the river god palace, however, was something he did not expect...

(Rough translation from Chinese summary, not entirely accurate.)