Alright. This review will be one big spoiler, so you brave little investigators can leave the room now. It's time for us Keepers to have a chat.
Ready? Good. A Dream of Japan reminds me of an episode of the Twilight Zone. Masterfully written, tense and suspenseful, and, more often than not, ending with the main character screwed over in some sort of dreadful fate in the end. Or like the horror movie where it only seems that our heroes win out, only that we know that the terribleness is just beginning.
And that's the drawback of A Dream of Japan. The ending has our investigators none the wiser that they really didn't succeed and are now pawns for the terrible horrible terror. Sure, actual investigators aren't supposed to defeat the forces of the Mythos, but the players may want to have that ridiculous concept of "fairness" in an adventure.
A Dream of Japan doesn't do this. Or, rather, it ends with the seeds for the Keeper to take his next adventure and drop hints (as well as NPCs close to the investigators) that their adventure in Japan isn't exactly over. It's a rather fiendish idea whose success will depend on how well your players don't mind being hornswaggled by the forces of the Mythos.
Otherwise, it's a great adventure. Chaosium set high standards of writing with its Cthulhu adventures, and A Dream of Japan follows quite nicely. It is, of course, set in Japan, yet its scope never has us needing a broad sourcebook to support it. The level of detail (at least until the end) makes our jobs easier.
Definitely an adventure to make your next one a little more interesting.
[4 of 5 Stars!]