When villages in Mexico are attacked by a strange winged creature,ornithologist and Elsie's old fiance Lawrence Cohen calls H.E.A.T. for help. They quickly discover the beast is Quetzalcoatl, an evolutionary hybrid of bird and dinosaur capable of breathing fire and protected by impenetrable feathers. The team must find a way to stop the creature, but matters worsen when Quetzalcoatl takes Elsie to feed its starving young.

H.E.A.T. heads for Scotland after Dr. Hugh Trevor (voiced by Roddy McDowall) of the Pisces Marine Research Institute claims his facility was attacked by theLoch Ness Monster. Once Nick is convinced of Nessie's existence, he discovers that the facility is hiding a terrible secret: Trevor has captured Nessie's baby and plans to sell it on the black market. Now the team must convince Godzilla to work with Nessie if they are to save her young and ensure her safety in Loch Ness.

As tension threatens to break the H.E.A.T. team up for good, major problems arise: a giant bat is terrorizing the countryside of Nigeria, the U.S. military have dredged the Leviathan ship from the bottom of the Pacific, and worst of all, the aliens are back, with an army of monsters under their control and plans to conquer the Earth.