Even though Godzilla Against MechaGodzilla is another stand alone film, it has its own continuity where Godzilla: King Of The Monsters is not the only film. Other movies such as Mothra, War Of The Gargantuas, and Space Amoeba (referenced in Godzilla: Tokyo S.O.S) are part of the film's time line.
The MechaGodzilla in this film is never called MechaGodzilla once through out the film and is referred as Kiryu instead. The reason is to make this MechaGodzilla stand out more than the previous versions.
After Godzilla's attack in 1999, the Japanese Self - Defense Force are forced to build a weapon that can outmatch Godzilla in a fight by using the bones of the first Godzilla.
Godzilla Against MechaGodzilla is one of the better movies in the Millenium series, but that doesn't say much. The Kiryu pilot, Akane, is considered a black sheep in the JSDF and she takes a journey in redeeming herself after she hesitated to attack Godzilla in her first encounter. Her story is kind of interesting because of her mistake and getting her partner killed, she is looked down on by her partner's brother, Hayama, and she tries to avoid conflict with him and redeem herself. The problem is that Hayama is the only character Akane really has a connection with. It is a shame that these two characters are not the main focus of the movie's story. I would have liked to see these two forced to work together more often so the story can establish them growing to respect and forgive each other in the end. While it sort of had that kind of story the way it was executed just made Hayama look like an asshole and they didn't really resolve their issues. Other than that she doesn't really have a good connection with the other characters and I think it's because of characters like Dr. Yuhara, the male lead character that helped build Kiryu, and his 10 year old daughter, Sara. Akane shows a lot of disinterest in Dr. Yuhara and you don't really see her wanting to get along with him. She actually connects better with Sara, but the way the movie tries to connect these three characters is really awkward. It really made me want to punch my TV from how Dr. Yuhara tries to get a date with Akane and fails miserably.
Godzilla goes back to the design from Godzilla 2000, but with some changes. In Godzilla 2000, his dorsal plates were more chaotic looking where in this film they look more controlled and smaller. MechaGodzilla looks completely different from the previous versions. It is the closest to looking like Godzilla and really looks more mechanical than the other versions. He has lasers that shoot from the mouth, missile launchers attached to the shoulders, laser canons attached on the arms, and Kiryu's a freeze canon called absolute zero. As cool as they look, the fight scenes are not that great. Call me crazy, but I just find the fight scenes boring and there is not much that happens in the movie. I was more invested in the story than what was happening with Godzilla.
Godzilla Against MechaGodzilla doesn't do anything new other than giving a new look for MechaGodzilla. The movie is more story driven more than the fighting aspect, but both kind of suffered equally for being kind of bland. If they worked on the story more, then it could have been a great movie. It's not to say that this is a bad movie, but it is mediocre. It is only watchable if you want to pass the time or if you want to watch this movie's sequel, Godzilla: Tokyo S.O.S.