Kong has been making the most of his time in Hollow Earth, though he has begun to feel a bit lonely. With help from Dr. Ilene Andrews (Rebecca Hall) and Jia (Kaylee Hottle)(who has been adopted by Ilene), plus conspiracy nut Bernie Hayes (Brian Tyree Henry) and Ilene’s old friend Trapper (Dan Stevens), Kong is led to a subterranean realm within Hollow Earth that is home to a tribe of Great Apes like Kong. However, they are being led by the ruthless Skar King, an evil ape that sought to lead his army to conquer the surface world until Godzilla imprisoned them within Hollow Earth.
While Godzilla has been keeping other Titans away from the surface to maintain his dominance, he has picked up a distress signal from Hollow Earth and goes to kill a powerful radioactive Titan called Tiamat to absorb her energy and charge himself for the incoming threat. When Kong is led to the Great Apes by a smaller ape called Suko, he battles Skar King until he finds that he is controlling a powerful ice Titan named Shimo, who severely wounds Kong by causing his arm to get frostbitten. Suko leads Kong to safety and brings him back to the humans. Trapper uses an old piece of his tech to create a glove for Kong to use to regain mobility and strength.
The humans work together with the surviving members of the Iwi tribe, whom Jia is descended from, to resurrect Mothra with Jia’s help. Kong goes to the surface world to get Godzilla’s help, but the great lizard tries to fight him again since he sees Kong stepping onto the surface as entering Godzilla’s turf. It isn’t until Mothra intervenes that Godzilla knows they have to join forces. The Titans return to Hollow Earth together just as Skar King and his forces arrive with Shimo. The Iwi create a gravity surge to prevent Skar King from entering the surface world, but in the middle of his and Shimo’s fight with Godzilla and Kong, they all end up going through the portal.
The final showdown takes place in Rio as Shimo’s ice powers are unleashed upon the land. Godzilla and Kong continue to battle Skar King and Shimo until Suko shows up with Kong’s axe and destroys the crystal that Skar King was using to control Shimo. No longer bound by her cruel master, Shimo assists Godzilla and Kong in Skar King’s beatdown, ending with Shimo freezing him and Kong breaking him into pieces.
Godzilla returns to Rome to rest at the Colosseum while Ilene and Jia appear to stick together with the Iwi. Kong goes back to the Great Apes with Suko and Shimo to become their new leader.
