SOG has its own single player storyline. It consists of 9 brand new maps (named SOG1 through SOG9) and shows lots of new monsters. You can start the storyline by pressing the "Play SOG story" button in the launcher.
Extract all files to Quake 2 directory and use shortcut to enable the game "console"
It's also possible to go through the original Quake II missions again, but this time playing as Blaze, Flynn or Axe ! It's also possible to convert the Quake II missions to a new style by setting the SOG_IDG1_ONLY, SOG_IDG2_ONLY or SOG_IDG3_ONLY sogflags.