WARNING: The following contains spoilers for Godzilla vs. Kong, now in theaters and streaming on HBO Max.
Ever since the first trailer for Godzilla vs. Kong dropped, fan theories have run rampant online about whether or not MechaGodzilla would appear in the film as the story's true antagonist. As the premiere of the film approached, the last marketing push confirmed the beast would indeed appear in the film to confront Godzilla. And with the advent of the latest iteration of MechaGodzilla, the robotic kaiju isn't just a manmade Titan specifically designed to take Godzilla down, but also provides a deceased monster with the opportunity to strike out at Godzilla from beyond the grave: King Ghidorah.
Ghidorah was revived in 2019's Godzilla: King of the Monsters by a group of international eco-terrorists led by the sinister Colonel Alan Jonah as a drastic means to undo the damage to the Earth that humanity had caused since they went dormant. While Ghidorah was ultimately destroyed by Godzilla during a fierce battle in Boston, a post-credits scene revealed Jonah secretly acquired one of Ghidorah's heads severed in an earlier skirmish in Mexico. In the time between King of the Monsters and Godzilla vs. Kong, the nefarious corporation Apex Cybernetics has acquired the head for themselves, unaware of just how corrosive the kaiju's influence is after his death.
While Apex's MechaGodzilla is piloted by Ren Serizawa, the son of the late Monarch scientist and Godzilla expert Ishiro Serizawa, the sheer complexity of the robotic monster's design requires an advanced neural network to operate properly. In order to accommodate for this, Apex has MechaGodzilla's controls interface with the brain patterns still present in the Ghidorah head. However, as MechaGodzilla draws power from energies within the Hollow Earth, Ghidorah's brain waves overwhelm Serizawa and the mechanical behemoth takes on a mind of its own as it ravages the Apex facility in Hong Kong, killing the corporation's founder and CEO Walter Simmons, before turning his attention to the nearby Godzilla.
Kaiju have always been depicted as living, breathing forces of nature far beyond the dominion of humanity as the most powerful creatures on the Earth. The incorporation of Ghidorah into MechaGodzilla's neural network puts a literal ghost in the machine that exists only to gain its revenge on Godzilla and destroy anything in its way. While the late, elder Serizawa worked in clear respect and admiration of Godzilla, his son likely blames the kaiju for his death in King of the Monsters and has his own deep-seated sense of revenge. This coupled with Ghidorah's insidious influence made MechaGodzilla an uncontrollable killing machine set loose on the streets of Hong Kong.
While Ghidorah himself doesn't make an appearance in Godzilla vs. Kong per se, the extraterrestrial kaiju's presence continues to haunt the world even years after his death. In attempting to develop a Titan of their own, capable of overpowering Godzilla and Kong, Apex accidentally manufactured the instrument of its own destruction in MechaGodzilla. Ren is the nominal pilot of the mechanical monster but, at the end of the day, it's Ghidorah and his corrosive hatred towards Godzilla and Earth that takes the reins of the robotic beast.
Directed by Adam Wingard and written by Eric Pearson and Max Borenstein, Godzilla vs. Kong stars Alexander Skarsgard, Millie Bobby Brown, Rebecca Hall and Brian Tyree Henry. The film is in theaters and on HBO Max now.