Three years after defeating Mechahodzilla, kong has established his new territory in the Hollow earth and searches for more of his kind. On Earth’s surface, Godzilla continues to maintain order between humanity and giant monsters, known as “Titans” — killing Scylla in Rome and resting in the Coloseum
afterwards.
A Monarch observation outpost stationed in the Hollow Earth picks up an unidentified signal. On the surface, the signal causes Jia, the last known survivor of the Iwi tribe from Skull Island to experience hallucinations and visions, causing her adoptive mother, Dr. Ilene Andrews, to worry. Also sensing the signal, Godzilla leaves Rome and attacks a nuclear plant in France to absorb the radiation. Godzilla then heads to the Titan Tiamat’s lair in the arctic. Monarch believes Godzilla is strengthening himself for an oncoming threat.
When a sinkhole opens near his home, Kong discovers an uncharted region where a tribe of his species has survived, including a juvenile named Suko. After an initial confrontation, Kong forces Suko to lead him to the tribe’s lair, and the two slowly bond on their journey. The tribe’s tyrannical leader, the Skar King, battles Kong with the aid of an ancient ice-powered Titan, Shimo, whom the Skar King controls with pain using a crystal. Shimo’s ice breath damages Kong’s right arm causing frostbite, but with Suko’s help Kong manages to escape, though losing his axe in the process.
Andrews and Jia, alongside Titan veterinarian Trapper and conspiracy podcaster Bernie Hayes, travel to the Hollow Earth to locate the source of the signal. They find the Monarch outpost destroyed. As the group ventures forward, they discover a temple that leads them to a subterranean section that houses a surviving Iwi tribe, who communicate telepathically with each other. Inspecting some ruins, they surmise that the signal was a telepathic distress call sent by the Iwi.
While observing Jia socializing with the Iwi, Andrews voices her fears to Trapper that Jia may choose to stay with her people and that Andrews would have to accept it. Inside a temple, Andrews uncovers hieroglyphics displaying the past and future: the Skar King once attempted to conquer the surface world and waged war against Godzilla’s species, but Godzilla defeated him and trapped him and his tribe deep within the Hollow Earth. The prophecy also indicates that Jia would be the key to reawakening Mothra Sensing Jia, Kong locates the temple and is fitted by Trapper with a prototype exoskeletal arm brace to strengthen and heal his frostbitten limb. Unbeknownst to them, one of the Skar King’s loyalists has followed them and informs the Skar King, who assembles his invasion force for Earth. Jia successfully awakens the reborn Mothra.
Godzilla kills Tiamat and absorbs cosmic radiation from her lair, evolving him and turning his dorsal plates magenta. Hoping to lure Godzilla to the Hollow Earth to assist him, Kong surfaces in Cario and calls out to Godzilla. Despite Kong’s attempts to communicate, an irate Godzilla attacks him, and a brief fight ensues until Mothra arrives with Jia and convinces Godzilla to help Kong. The trio then head down to Hollow Earth to face the Skar King along with Shimo and his army. The fight drags Godzilla, Kong, Shimo and the Skar King to Rio de Janerio, where Shimo begins to induce another ice age at the Skar King’s command. The factions are evenly matched until Suko arrives with Kong’s axe and destroys the crystal controlling Shimo. Shimo and Kong then freeze and shatter Skar King.
After undoing Shimo’s ice age with his atomic breath, Godzilla returns to rest in the Colosseum. Jia reunites with Andrews and alleviates her fears by choosing to stay with her adopted mother. Mothra restores the protective barrier of the Iwi’s home and then departs. Kong returns with Shimo and Suko to Hollow Earth, where he becomes the ape tribe’s new leader.
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