A flying saucer goes past Jupiter and to Earth. A farmer on a tractor sees the saucer, which hovers above him. He tries to run but it freezes him. Human looking aliens emerge. They have back outfits with a dragon emblem on the front. They take the farmer back to the ship and put him in a room. A naked woman enters. After, he tells the sheriff about his abduction and how he has sex with an alien.
Professor Duncan (Robert Vaughn) is on a radio show talking about the existence of UFOs. He explains that the most important scientific discovery in the last part of this century will be the answer to UFOs. Back at his office he gets a call from the abducted farmer and goes out to meet him. There's a big radioactive circle where the saucer had landed.
On the ship Captain Ramses (Christopher Lee) explains that their planet's sun may explode and that a migration has been planned as soon as they find another planet. Earth seems well suited. He tells the other crew members that they need a human female to study--so they abduct a family driving in a car. Once back in the vehicle they have no memory of what happened. But soon after, the family kill themselves.
Ramses says that the examination of the humans prove that their ancestors were transplanted humans, which he obviously does not like. So he wants to kill all native humans and take over the planet. The only thing standing in his way is that the Intergalactic League of Races (which includes friendly, big headed aliens) has a pyramid base on the bottom of the ocean and they can't know of their evil plans. So he goes there to take care of this potential problem. He parks his UFO among other space vehicles and is greeted by a robot, Durbal (who later saves the day)--and taken to the base leader, a big headed bald alien. He says that Earth and its inhabitants are protected by the Galactic Treaty. Ramses has sex with a red-headed woman then goes to the communications room where they are monitoring Professor Duncan's appearance on a TV show. As this is happening one of Ramses' people, a dark haired sinister looking woman, sabotages one of the bases' flying saucers so that when it goes in the atmosphere and is spotted by the Earth military it can't outmaneuver the missile or put up its force field. Then, they seize the base. They do meet some resistance and one of the base people warns their main ship, which is now damaged. They need help. So they abduct Duncan and explain the situation to him, of how their base was taken over and a renegade group wants to exterminate mankind.
Meanwhile, the evil alien fleet begins to arrive in Earth's orbit. They have a mind control device that can make people kill each other.
The good aliens need computer parts to replace the damage on their ship and Duncan can help them. There's some really dumb dialogue in which he says "If I do what you want will you return me to my family?" Well, if he doesn't do what they want he won't have a family anyways!
The movie ends with a battle in space. The tide is turned when the good aliens manage to activate Durbal the robot (yes, STAR WARS came out the year before) and have him take the base back. He is able to destroy Ramses' ship that is making people commit suicide. Ramses fleet is defeated and rather than surrender he crashes into the moon.
STARSHIP INVASIONS is a very entertaining alien movie, though it loses some of its steam towards the end.