From Destinypedia, the Destiny wiki
Six is the ninth story mission of Destiny 2, taking place between Looped and Sacrilege, and is the last mission on Nessus. Completion of the mission will unlock all 7 Adventures on Nessus: Hack the Planet and Deep Conversation. Release and Lost Crew, Unbreakable and Exodus Siege, as well as the Invitation from the Emperor.
During the course of the mission Six the Guardian finds the Vex teleporter in which Cayde-6 is trapped and manages to shut it down, freeing Cayde. However Cayde promptly comes under fire from the Fallen who have followed him and/or the Guardian to Nessus, just as they followed the City forces led by Commander Zavala and Sloane to Titan[note 1], and have now been ambushed by the Vex while attempting to scavenge the Exodus Black. The player fights through these two warring parties to reach and rescue Cayde who has taken cover from the Fallen inside the Exodus Black. The player defeats the Fallen besieging Cayde and the AI Failsafe, meets with him, learns what he was doing on Nessus, convinces him to rejoin the Vanguard and learns where Ikora Rey went after the fall of the City.