On the island of Malau, marine biologist Jack Ellway (Adam Baldwin) studies the effect of seismic activity on the area's marine life. A string of mysterious drownings soon turns their research project into a beachfront disaster as Jack learns that a nearby underwater trench is home to a family of giant amphibians that have mutated after prolonged exposure to buried pesticides. As Jack and local doctor Alyson Hart (Julie Carmen) explore the surrounding ocean for the creatures, the military comes and tries to investigate. Jack's son, Brandon (Emile Hirsch), discovers and befriends a baby creature that is able to leave the ocean and walk on land. A more dangerous, bear sized creature responsible for incidents at the beach and implied to be the baby's older brother, also comes ashore and causes a panic before being captured.
Whilst held captive, the older brother and baby creature's cries cause the mother to rise from the deep to reclaim her children. After a plan to lure the mother back out to sea by releasing the younger creature goes astray due to a greedy fisherman trying to abduct the juvenile for a huge payout to an exotic animal dealer, the mother begins searching the island for her offspring. Her destructive advance forces Colonel Wayne (Bobby Hosea), in charge of the local the military, to get involved. Fearing to the safety of the islands and his men, the Colonel reluctantly gives the orders to attack back and the mother creature is killed. Revealing the existence of the baby creature to his father, Brandon assists Jack in a new plan to ensure the creature's safety whilst the remorseful Colonel Wayne buys them time. An adult male, the mate of the slain female and father of the two juveniles, arrives to reclaim his offspring and deceased mate. The Ellways manage to free the two juveniles and all three remaining creatures depart back to the deep sea.
Gargantua made its television premiere on the same night that one of its rival projects, Godzilla (1998), had its theatrical release.[2]Gargantua holds a 21% rating on Rotten Tomatoes.[3]
Julie Carmen was nominated for the ALMA Award in 1999 for her performance in this movie. She was indicated in the category "Outstanding Individual Performance in a Made-for-Television Movie or Mini-Series in a Crossover Role".[4]