Rosso and Viola pick up Irvine and Van before they are hit by a charged particle beam from the Death Stinger and take them to the Ultrasaurus, which is out at sea. Fiona and Moonbay greet Van, Irvine, and Zeke when they arrive while Rosso and Viola fly off. Moonbay says that this is her Ultrasaurus. Hermann and many other soldiers welcome Van back. Later at a meeting, Hermann explains that they need to go to an Imperial base to pick up the most powerful (and largest) weapon ever made, the gravity cannon. Then Van and Hermann visit Krueger, who is recovering from injuries and stress, and they talk about how he and Van's father were best friends. Meanwhile, Rudolf arrives at a secret base where the gravity cannon is and meets Dr. D, who has been working on it. Dr. D says that they have only been able to make three shells for the cannon.

A Hammerhead on patrol is caught in an explosion by some icy gas released from the bottom of the sea. At a meeting, it is explained that the icy substance is methane hydrate. It can be lit by fire, so it is also called "fire ice", and then it releases the gas that made the Hammerhead sink. To avoid it, they are going to take a longer but safer route. Then an alarm rings signaling that enemy sinkers are approaching. Everyone gets into Zoids to fight, Van, Irvine, and Thomas in their Zoids, Moonbay taking the controls of her Ultrasaurus, and Karl in his Iron Kong. They all fight the Sinkers and start to destroy them. Meanwhile, the hidden base Dr. D and Rudolf are at is attacked by a bunch of enemy Zoids. They are thinking about abandoning the gravity cannon when Rosso and Viola arrive and destroy all of the enemy Zoids, saving the day once again.

The Death Stinger is detected approaching the Ultrasaurus. All of the Zoids are called back and the Ultrasaurus prepares to dive. They barely manage to get underwater to avoid the charged particle beam from the Death Stinger, which can fire it above water, but not below. Hermann comes up with a crazy plan to go to the place where they found the methane hydrate and use it to fight the Death Stinger. The Death Stinger reaches the Ultrasaurus, shoots it, and then starts to tear through the armor with its claws. Depth charges are released by the Ultrasaurus and they explode on the methane hydrate, which causes icy gas to rise up and explode when it touches both of the Zoids. The Death Stinger is forced away by the explosions and the Ultrasaurus is able to continue on to get the gravity cannon.