Reunited is an unmarked quest that kicks off the questline.
The questline begins as soon as the player travels to a level 15 or higher system - assuming no other higher priority events (such as the start of Shattered Space) kick in. It kicks off with a random space encounter where a set of Watchtower Ships appear and hail the player.
Whatever the options the player chooses in dialogue - to submit, fight or ignore - the player will eventually be hit with an unknown weapon which will unpower their ship and eventually fade them to black.
The player will wake up strapped to a chair on another Ship; two Watchtower Agents will be talking about them. A scene will play out resulting in the introduction of Torin who will rescue the player and ask them to meet him at Stardock.

The next quest starts immediately from here.