When a strange electrical creature known as the Crackler attacks seemingly random locations in Manhattan, the team investigate. But the Crackler isn't behaving like the mutations H.E.A.T. have fought before, and what is its connection to mild-mannered insomniac Sidney Walker?

The team are called to help a rescue mission for xenobiologist Alexander Preloran, who disappeared while on a mission to explore the Leviathan, an alien spacecraft buried at the bottom of the Pacific for 65 million years. But once they go down, the team discover the ship's alien crew are very much alive... and they have plans for Earth.

H.E.A.T. investigates the tropical island of Santa Marta in the wake of a devastating volcanic eruption. But what they find on the island is that, due to irradiated lava, a hideously mutated ecosystem has developed, including carnivorous plants and gigantic bees. As the volcano begins to erupt again and Godzilla confronts the bees' queen, the team must find a way to escape the island alive.