In addition to the main game, for those that buy Tengoku - hen at launch they will also get a separate downloadable sidestory game called Rengoku - hen. (mechadamashii.com)
Certain characters have optional sidestories within Arkham city (the article mentions Zsasz), that you can dig into. (xboxic.com)
Natalie Portman's directorial debut was meant to inspire others by sharing the life story of Amos Oz, a famous Israeli storyteller; but it gets dragged down in dreary sidestory ultimately fails to invigorate the audience... (ff2media.com)