I am kind of surprised to see such low rating and bad reviews.
To be quite fair, I found the movie a fantastic entertainment. I hardly ever laugh so much during the film as I did yesterday.
First of all, I would not say that plot is nonexistent here. It is pretty chaotic, but that was the point here, and this is where the charm of Turtles lays.
I see it in a way as a parody of the detective movies with a simple plot (a crime happened, police is chasing them, the criminal is caught). Here we have no crime, the whole spying looks a bit surreal until the very end, the detectives talk nonsense most of the time, the clumsiness of the investigation is hilarious.
Many scenes look just random, but again this is why they are funny. Dancing hairdresser or the lottery with 'fabulous' prices might not add much to the development of the spy plot, but they help to create a great impression of a crazy life the main character is leading. It is the accumulation of the surprising elements and the unexpected that put the smile on my face.
You can also find bits parodying teen movies (Suzume crush on Katou, and their rendez-vous years later), and once again, I could not stop laughing how awkward, but how close to real life that was.
All in all, I think it is really worth giving it a chance, just sit back and relax, and if you don't expect a highly intellectual film, you might love it as I did.