River Monsters was cancelled because they ran out of monsters! They literally caught them all (Note: This is the real reason; I'm not making that up!).

Because of this—and the impressive staying power/popularity of the show over time—the executives have decided to fund continuing research to discover a new world, and this research has finally bore fruit!

A new world—full of rivers—has finally been found! Because of their continuous funding, they get first crack at it.

A daring crew is assembled, they start recording, and enter the portal...