Spirou and Fantasio, swallowed by the raging waters of the river, find themselves in an unknown valley filled with very peculiar, and very dangerous, life. As they brace themselves to face a hostile nature long enough to find their way out, Fantasio contracts a deadly disease that turns him into a homicidal maniac. Spirou must now find a way to save him and himself and escape the deserted valley. If it really is deserted, that is...
