Director Noriaki Yuasa's 1969 film was originally called Gamera tai daiakuju Giron, but it was dubbed into English as Attack of the Monsters.
Some kids see a flying saucer land, and two of them venture inside. Probably not a wise decision -- didn't their mothers ever say don't go inside a strange flying saucer without an adult present?
Fortunately we don't have to deal with aliens who want to probe them in their bathing suit areas. But the flying saucer takes off, carrying the two kids into space. It looks like they're going to collide with a space rock when suddenly a giant flying turtle that spits flames out of its ass arrives and saves the flying saucer and its occupants. Yes, it's Gamera to the rescue!
The kids arrive on a planet, and witness two other monsters slugging it out -- one is a giant bird, and the other looks like a giant lizard with a big axe blade for a head.
The kids sneak into a building and find the aliens that brought them to the this strange planet. They seem to posses knowledge far in advance of our own. You can tell that they are scientifically advanced beings because they wear fancy helmets and capes. But they aren't so advanced as they think -- they continually interchange the words "planet" and "star."
The lizard with the axe for a head is Giron, and the kids have been pawns in the aliens' plot to lure Gamera into a fight with Giron. Then the aliens decide that they'd better eat the kids' brains, just to be on the safe side. And they get pretty close, too. They shave the first kid's head and have the bone saw practically biting in to the kid's flesh when Gamera returns to slug it out with Giron in a violent battle. Giron wounds Gamera, who spurts buckets of green blood. But Gamera recovers and eventually saves the day by sticking a rocket up Giron's nose and blowing him in half.
I've never followed many of the Japanese monster movies, but I've seen the occasional Godzilla film, and the of course the original Gojira is a bona fide masterpiece (with or without Raymond Burr). But this is nothing but kid-friendly foolery. The monster costumes and special effects are awful! Frankly I think even kids would find this silly.