Halo 4 marks the return of John-117 - the "Master Chief" - as a playable protagonist for the first time since 2007's Halo 3, as 2009's Halo 3: ODST and 2010's Halo: Reach focused on new characters and playing styles. Having been lost in space in the finale of Halo 3, John-117 and Cortana, adrift aboard the wreckage of the UNSC frigate Forward Unto Dawn, find themselves near a mysterious Forerunner planet, which was first seen at the end of Halo 3 once players beat the game on the highest difficulty setting. A key plot point will involve changes to Cortana and how her exposure to Halo and her operation beyond her natural lifespan may have contributed to her becoming "rampant." Source: Wikipedia, "Halo 4", available under the CC-BY-SA License.