A Class M planet, Nibiru was the home world of the Nibirans, a technologically unsophisticated race of humanoids. In 2259 the USS Enterprise was on a mission to survey the planet when they discovered that it threatened with an environmental catastrophe as a result of a gigantic volcano which was near to eruption. As this would have rendered the Nibirans extinct, Captain Kirk chose to intervene to avert the eruption and prevent the disaster. 
Afterwards, Kirk lied about their actions on his report to Starfleet. Spock, however, chose to relate all of the details of their mission. Admiral Marcus responded be relieving Kirk of his command.