As a follow-up to one of Toho's lesser attempts at the giant monster genre called Frankenstein vs. Baragon, the sequel War of the Gargantuas ends up being one of their all time best. Plagued by a few story issues which are lost in the background, this wild, action oriented monster epic is a classic, right up there with the studio's Godzilla efforts of the time.
For a sequel, War of the Gargantuas does a miserable job of explaining itself. Not only do the rampaging beasts bear no resemblance to the Frankenstein monster of the prequel, their origins are poorly explained. Russ Tamblyn takes on the role of the lead scientist and is used in flashback scenes as well. This might not be so noticeable if Tamblyn looked at all like Nick Adams, who took the role previously. Kumi Mizuno reprises her role, making Adam's absence more glaring.
All of that is forgotten or ignored quickly though. This is one of Toho's fastest paced epics, rarely stopping to catch up to itself. The opening scene alone contains more action and scare tactics than a lot of their films combined. One giant squid versus giant unexplained green monster fight later, the movie begins to introduce its story slowly.
As disasters happen, the scientific community who took care of the Frankenstein project is grilled, defending their beast as harmless. Given that the creature looks nothing like the one in the previous film, it's hard to imagine the media would even think this was same monster anyway. Regardless, this leads some epic battles between the military and this new creature.
This was the first movie to feature the use of Maser tanks, laser cannons firing electrical beams that simply look fantastic on film. They later became a staple of their films. Mixed in the additional military might of tanks, planes, and soldiers, this brings constant action. Miniature work is simply incredible aside from a few shots that end up being too close to the camera to withstand scrutiny.