It's almost, but not quite "plain". You have to make sure that the mimetype file is the very first file in the zip, and that the mimetype file is uncompressed.

It's almost, but not quite "plain". You have to make sure that the mimetype file is the very first file in the zip, and that the mimetype file is uncompressed. Otherwise the epub will fail validation and will not work on many readers. So yeah, you can unzip them with any standard zip utility, but zipping them back up requires some care.