I think other than Super Monster, this is the worst film in the Showa Gamera series. However, re-watching it recently in HD made me realize there are some things that make it worth watching and to compliment it for.
The main problem, which also makes it unique, is how little plot there is, and how little (original) monster mayhem there is compared to other Gamera films. Jim and Masao fool around and meet Gamera and are subsequently kidnapped by aliens. The aliens use them as hostages to control Gamera to take over Earth. Gamera goes on a rampage, but then the kids escape and free him. Gamera attacks and in a last ditch effort to beat Gamera fuse and form Viras. Much of the film is Jim and Masao flaffing about, either on the outside with their Scout Troop or inside the Virasian UFO, and the middle section features two-stock-footage heavy sequences with footage from all three of the previous Gamera films. Add to that the already short runtime (81 minutes or 71 minutes depending on the version, and you've got a pretty barebones film. Add to that very few new miniatures and relegated all of the scenes in cities to aforementioned stock-footage, and the film can feel very cheap and isolated.
Viras, the titular monster, unlike Gyaos or Barugon, isn't a major presence in the film and only shows up in the last ten minutes. Previous Gamera films had Gamera fight his foes at least twice (Barugon x2, Gamera x3). Here, Viras gets little build-up; just two short scenes showing him in a cage, before he's unleashed. The film also seems to be unsure exactly what it is; at first it seems like the film is setting up the lead Viras is a strange space-creature to that the Virasians captured, but then it's reveled to be the boss of them. It's weird and doesn't make a lot of sense. You'd think that maybe Viras was an alien separate from the UFO, a beast brought to Earth incase things go wrong, but instead it's their leader chilling out. Still, when Viras is unleashed it's a sight to behold.
Viras is a fantastic monster and sky-rocketed to one of my favorite Showa-era monsters after this viewing. To begin with, the suit is impressive. According to Wikizilla, the suit was made with a very flexible plastic, Urethane, which wasn't often used in the construction of suits. This coupled with the monsters blue color and squid-like appearance make it standout against all the other reptilian and quadrupedal Gamera foes. Even though it's a "man in a suit", and it's obvious in a few shots that it's being held up by two "main legs" and when looking at images of Viras directly, the film does a lot to hide this and make Viras way more squid-like. There's moments where we see only Viras's tentacles, or a puppet and other props of it, and the other tentacles move around too, making it feel much more squid-like an alien. It's frightening face, and savagery it displays in the fight make it memorable. Viras, unlike Barugon and Gyaos, doesn't have many unique abilities: all it really does is attack with it's tentacles and spear-like head. The scene of Viras impaling Gamera is super gory, even for a Gamera film, and it's painful to watch.
It's a shame the film doesn't live up its poster. While I understand there were probably budgetary reasons, I would've liked to see Viras a bit more before the final climax, or at least have the Virasian UFO destroy some cities itself directly. A big problem with the film is that it's main conflict, Gamera destroying cities, is entirely composed of stock footage.
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They're...fine! They're silly children who like playing pranks. The actors in the Japanese version do fine enough, and contrary to genre stereotyping aren't annoying. Masao's defining trait is that he's an inventor, and Jim's defining trait is that he likes to take photos and has a lasso. All of their skills play roles in the story. The moment where Masao tries to trick the computer by ordering fruit, hoping it would give them a knife, is a silly but clever way to show his ingenuity. I suppose some people might blame them for switching the submarine controls, but the film is operating with kid-logic, where adults allow kids to take rides in submarines, so it's a non-issue if you just let it be.
Other than them, there's really not much going on the human side. The adults and backdrop of the troop don't serve any major function except to show who is worried about Masao and Jim getting kidnapped. It adds some humanity to Jim and Masao and makes the film seem far less empty. It also leads to one of the most dramatic emotional scenes in a Kaiju flick; debating whether to sacrifice Masao and Jim for the greater good. Really that scene with Masao and Jim telling the people of Earth to not surrender and attack even if they'll be killed, is one of the realest scenes in a film like this, surprisingly selfish for two child characters. It's a great scene, and does add a lot of emotion to the narrative.
Finally, I want to talk about Gamera. The scene with Gamera, Jim, and Masao underwater is magical. The lengths Gamera will go to save the boys is also magical. Some people won't like this turn for the character, but I kind of dig it now. There's this force of unbridled positive energy, that isn't always honest, but I like it as a fantasy. This was the film that turned Gamera really into a hero of the children and every film from here on out would be very child-oriented. It's a weird paradox though where this film Gamera appears a lot, but a good chunk of it is stock footage. Still, loved Gamera in this film.
Overall, I think this film is FINE. I used to think it was trash, but I see a lot to appreciate about it. The emotional core of Gamera and the children is great, Viras is cool, and the antagonistic alien force is creepy as well. It's a shame how reliant it is on footage from the past films and how simple the plot is, otherwise it would probably be just as good as the previous entries.