In 2006, twin sisters Madolyn and Quinn Ramsey view Snakes on a Train, which was mistakenly purchased by their grandmother Lucy Taggart, who believed it was Snakes on a Plane. The sisters bond over the film and develop a love of The Asylum's movies.
Over the next two decades, Madolyn becomes a commander in the United States Navy while Quinn Ramsey, now at NASA, and the sisters grow apart, their paths diverging as life takes them in different directions. A dramatic turn occurs when Madolyn’s ship is sunk by a giant piranha, leaving her the sole survivor; she later stumbles into a city under attack by a pair of colossal robots and manages to seize control of one, shifting the balance of power on the battlefield.
Meanwhile, Quinn discovers that the International Space Station has been destroyed by a rogue asteroid and informs their father Thomas, a Cabinet official. As the world tumbles into chaos, strange drones appear in multiple cities, scanning areas with lasers; Washington, D.C. is struck by a giant crocodile, amplifying the sense that something larger-than-life is at play.
An attack on a subway in Tokyo by a giant snake pushes Madolyn and Quinn to recognize a chilling pattern: the creatures mirror those featured in The Asylum’s films. The menacing phenomena spread further as giant sharks surface off the Eastern Seaboard. The FCC’s investigation uncovers suspicious activity around Asylum titles on streaming services, where certain films have been streamed for unusually long periods.
Researchers and military officials conclude that a hostile alien race has misread The Asylum’s creature-driven mythology as real history and is using facsimiles of those beasts as weapons to invade Earth. The drones 3D print new creatures, but because the aliens’ home planet receives only a weak streaming signal, the facsimiles come through in low resolution and fail quickly. Humanity deduces that these beings can be fought by tactics borrowed from the original films itself, leading Thomas to orchestrate a daring plan: lure a Mega Shark into battle against a five-headed shark sent for another attack.
The aliens quickly realize that humans have deciphered their strategy and respond by sending a ship to land on Earth. Quinn and a military unit trace the location of a damaged drone, but the squad is nearly overwhelmed when some soldiers are brainwashed by the aliens using lights projected from their hands. Quinn evades capture and secretly boards the alien ship. The mothership’s location is identified, and submarines are refitted as Mecha Sharks to spearhead the assault.
Madolyn reconstructs the Armada Suit she acquired earlier and joins the fight, piloting it against a brutal hybrid creature forged from multiple beasts. The aliens respond by summoning a sharknado to shield the ship, unleashing a chaotic whirlpool of danger. While Madolyn battles the sharknado, the brainwashed soldiers return to the bunker where Thomas is sheltering, intent on converting the President to push the invasion forward. A loyal soldier who avoided conversion warns Thomas of the brainwashing’s reach, and Thomas makes a valiant effort to escape while protecting those taking refuge with him — only to be captured and transformed.
Madolyn’s Armada Suit sustains heavy damage, but the battle rages on. Quinn is discovered by the aliens and nearly converted, yet she exploits their weakness to soundwaves and escapes on a landing ship, then sabotages the drones’ control system. As the mothership attempts to leave Earth through a wormhole, random creatures surge into existence, including a giant octopus that attacks the vessel. The sisters’ paths converge when their lander crashes near Madolyn’s location, and they reunite to face the brainwashed, former humans now led by Thomas. Yet Thomas retains a shard of humanity and leads the group away from total transformation. United at last, Madolyn and Quinn acknowledge the depth of their estrangement, and now, more than ever, realize that their fight is far from over, signaling that their struggle against this alien threat has only just begun.
