Synth Retention

Synth Retention is a quest in Fallout 4.

Quick walkthrough
Help Father in retrieving X6-88, a rogue synth.

Detailed walkthrough
Speak with Father about The Institute, he'll request help dealing with a renegade synth turned raider at Libertalia. There will be a Courser companion at Nahant Wharf who needs to be collected first. Once he is collected, fight along the network of ships until the target is reached. On entering the cabin. X6-88 will initiate conversation and provide Gabriel's recall code.

On reaching the top of the ship Gabriel will initiate conversation, no matter what a fight will break out, but there's a couple of persuasion checks that give extra choices like the password for the Master locked terminal that can open a nearby safe. Once the fight is over X6-88 will teleport out with Gabriel.

On returning to the Institute there will be another discussion with Father, he'll provide quarters upon entering them the mission is complete and The Battle of Bunker Hill begins.

Bugs
X6-88 may not appear next to your quarters at the Institute, preventing the final objective from being completed. To fix it, fast travel to Libertalia and speak to X6-88. Alternatively, open the console and type "prid 000e210a" followed by "moveto player" while standing in front of your quarters, which will force X6-88 back into his normal routine.