Plot:
A spaceship is orderd to conquer Earth. Things would be in bad shape if not Gamera flies up and destroys the ship.
In Japan, at a boy scout camp, Masu and Jim, the 2 troublesome scamps sneak off and tinker with the controls of a mini-sub by wiring them in reverse. They're found later. The scouts are supposed to pilot the mini-sub and are able to talk the Troop Commander and the sub designer into letting them take it for a test run.
A second spaceship arrives and must locate Gamera.
The sneaky kids spot Gamera while in the sub.

Gamera: Isn't a submarine supposed to have some kind of killing like a Spam can?
The ship finds Gamera and uses some energy beam to hold Gamera. The aliens decide to read Gamera's mind to learn of his powers, birth, and
weaknesses. If anyone thinks this is the best way to replay footage from other Gamera movies like Gamera fighting Barugon, the ice monster, and Gaos, the supersonic monster, you deserve a promotion. The aliens then realize that Gamera has an overpowering kindness towards children. The energy beam wears out, and Gamera flies off.
The ship catches Masu and Jim. The aliens then contact Gamera and threaten the boys if he attacks their ship. The boys look around the ship and
find the aliens preparing a mind control device for Gamera.

Who says I had enough energy drinks?
The boys keep snooping around the ship which don't bother the aliens.

Scouts roaming around a spaceship aren't going to get into any trouble...
The kids find a alien-squid-Cthulu monster caged up before, they're captured by the aliens.
The aliens order Gamera to start destroying various things on Earth including a dam and a city. Finally we get some more sweet Kaiju
destruction. The aliens demand that Earth surrender because they control Gamera by threatening the 2 boys. If Earth doesn't surrender, the boys will
be killed.
The military is waiting for orderes from the UN to decide whether or not to attack or surrender.
The boys escape the alien's straps.
The UN orders to surrender when the Troop commander is able to raio the boys and convince them to rewire the circuits on the Gamera mind control
by setting them in reverse. After doing that, the boys beam back to Earth.
Unaware, the aliens order Gamera to kill the boys. However, since the friend to all children is freed, Gamera attacks the ship. The aliens report to the alien-squid-Cthulu leader that absorbs the underlings and grows into a giant-squid-Cthulu-alien.

I'm a giant monster squid before lame CG effects ruined us all...
Finally, Gamera versus the giant-squid-Cthulu-alien battle.

Call 911, Gamera's having deep stabbing chest pains...
The wounded Gamera flies up with his giant flying rocket turtle trick as the giant-squid-Cthulu-alien falls off and into the ocean.
Gamera flies off as all celebrate.