This feature version of Johnny Sokko and his Flying Robot (1967) is edited together from four of the show's episodes--the first and last and two from the middle.
A weird UFO enters Earth's atmosphere and is pursued by aircraft and crashes into the ocean. But before it does we see it's piloted by a hideous alien named Emperor Guillotine. Then we are introduced to a young boy, Johnny, and Jerry Mano, who are on a cruise ship in this same area. A monster named Dragulon emerges from the depths and sinks the ship. Johnny and Mano get washed ashore to a nearby island where they are captured by the evil Gargoyle gang. It's then it's revealed that Jerry is an agent of Unicorn organization, which protects Earth from threats. They are helped by scientist Lucien Guardian, who shows them his creation--Giant Robot. He explains that the mechanical man will be under the control of the first voice he hears in his electronic brain. The scientist then helps them escape--and sets off a nuclear bomb that destroys the enemy's base. The robot survives--and is now under the control of Johnny.
The next monster that appears is Nucleon, which looks like a giant dog toy. Meanwhile, Johnny is taken by Mano--whose agent name is U6--to Unicorn headquarters an is made an agent because Giant Robot is the only weapon capable of defeating Gargoyle's new threats. Next, there's a leach snorkel monster, "The Vine Monster", that drains all of the water out of a lake. We're introduced to two more aliens, one silver, one with a peg leg that looks like the creature in FRANKENSTEIN CONQUERS THE WORLD. The lead scientist of Unicorn tells the government not to bomb the Vine Monster because the energy from the explosion will cause it to grow even larger. Of course, they don't listen and bomb the creature.
Then, a canine-like bipedal monster swallows a train. The alien agent this time looks like Gene Simmons with an eye-patch. Gargoyle wants to stop the world Unicorn meeting. A scientist is brainwashed to be an agent of Gargoyle--but in the end they rescue him.
Johnny gets shot and is in the hospital, where Gargoyle tries to kill him. Giant Robot fights a Giant Eyeball. At the end of the movie Guillotine transforms into a giant and battles Giant Robot. But if the evil alien leader is killed there will be a nuclear explosion. So Giant Robot, disobeying Johnny's commands, flies into space and crashes them into an asteroid. There's a huge explosion--and both are dead.
This is a very entertaining giant monster movie with a surprisingly downbeat ending.