Pacific Rim is a movie about giant robots fighting giant monsters. It's Godzilla meets Robotech or Voltron, with some Halo thrown in. What more do you need to know?
In the prologue (which could have been an entire movie by itself), we see the first giant monster come out of the sea and attach San Francisco. It takes several days for mankind to destroy it, and thousands of people are killed. Then more monsters start to appear. The monsters are called Kaijus, and the humans build giant robots to fight them. The robots are called Jaegers.
When the movie proper begins, it's many years into the Kaiju war. The Jaeger program is about to be scrapped and mankind is losing. The monsters are getting bigger, stronger and smarter. The commander of the Jaeger forces, Stacker Pentecost (Idris Elba) tracks down a former pilot who's now working construction. He convinces the pilot, Raleigh Becket (Charlie Hunnam) to return for the final battle. Becket is haunted because his brother was killed while the two were battling a Kaiju.
There is a good half hour or so where not much happens. The characters are learning about each other and I almost dozed off. The dialogue is not great, and there is a lot of time spent arguing about whether Mako Mori (Rinko Kikuchi) should be allowed to pilot a Jaeger.
Once the action gets going though, the movie is a lot of fun. Unlike the Transformers movies, director Guillermo del Toro does a great job of not using too much shaky cam. The huge battles between the giant monsters and giant robots never got too confusing or overwhelming.
There is a bit of comic relief provided by Charlie Day as a scientist, and I also enjoyed the scenes with Ron Perlman as a black marketer who sells Kaiju parts. The movie isn't that original but it was done well. I just wish it had better acting and dialogue in the early part of the movie.