Aquatic ecologist Dr. Zeb Hogan travels to Spain in search of the invasive wels catfish - a monster fish capable of growing over eight feet long. Biologists fear this voracious predator could be destroying the local ecosystem. Teaming up with local fishing guides and scientists, Zeb investigates why this beast has flourished in Spain's Ebro River and if indigenous species have declined as a result. Zeb also makes a stop in Germany, where he meets the man claiming responsibility for smuggling and releasing the invasive into Spain. Before his mission is complete, Zeb has just one last thing to do: catch one of these monsters himself.