In Mega Shark vs. Mecha Shark, the titular Mega Shark returns and attacks a submarine, causing it to sink to the ocean floor. The world's militaries go on high alert, as everyone fears the unstoppable beast. As Mega Shark wreaks havoc on ocean traffic, the US government activates the Mecha Shark project, which is a top-secret program that has developed a mechanical shark designed to have the same characteristics as Mega Shark.
The Mecha Shark is piloted by a pair of scientists, Jack Turner and Rosie Gray, who are tasked with leading the fight against Mega Shark. At first, the Mecha Shark is successful in battling Mega, but a malfunction causes the robot shark to turn on a nearby island where civilians are located. The scientists must now find a way to lure Mega Shark to Mecha Shark to fight each other to the death.
Furthermore, the movie has a subplot involving the pilot of the sunken submarine, who works with the scientists to try and take down both Mega and Mecha. The film also features various other incidents involving Mega Shark and how it affects the world, such as attacking a passenger plane and destroying cargo ships.
Throughout the movie, there are intense action sequences as well as moments of humor and cheesy special effects, which are hallmarks of the Sharknado film franchise director, which also produced Mega Shark vs. Mecha Shark. The movie falls into the B-movie category, and while it was not a critical or commercial success, it gained a cult following amongst fans of the genre.