Best Left Forgotten

 is a main quest in the Fallout 4 add-on Far Harbor.

Synopsis
To defuse the tense situation on the island, DiMA requires his oldest memories, stored at The Nucleus. Problem is, the Children are on a war path and this will require subtlety. The Survivor is sent in to hack into DiMA's memories and retrieve them.

Walkthrough

 * 1) Travel to The Nucleus.
 * 2) Complete Visions in the Fog to gain membership.
 * 3) Enter The Nucleus's command center from the door inside the submarine pan, at the far end at the top.
 * 4) Navigate the gauntlet. Make sure to watch out for the Assaultron dominator that guards the command center.
 * 5) Throw the switch in the back to activate power. Mind the above.
 * 6) Enter the virtual reality. The rules are fairly simple: Destroy the firewalls using the green ray (aligned using the usable cubes with crystals inside, they redirect the laser), built a direct path to the orange column for your indexers (the little green worms, they are your best buddies), and defend them from sentry counter-measures using turrets. You get five turrets, but their range is very long, so place them with long fields of view. Don't bother picking up after yourself, the inventory is reset each time. To pass, you need five memory pieces to be retrieved.
 * 7) For stage one, just follow the tutorial. Build the path as you go and place the turrets on the walls.
 * 8) For stage two, the firewall disabling ray is just under the starting point, obscured by a code block. Later on it's a matter of aligning them. Spread out the turrets. The central corridor can be safely left uncovered.
 * 9) For stage three, note that the ray aligner is above the left-hand firewall. You can easily jump up there from the top.
 * 10) For stage four, you need to first disable the firewall on top, then remove the code blocks you used to align it and send the beam along the lower level.
 * 11) For stage five, route the beam around the arena. Your goal is to place it on the level of the last firewall, using the relays found above the left-hand platform.
 * 12) With the memories collected, gather the pieces: The nuclear key, killswitch code, and inspect the Vim! factory.
 * 13) Enter Harbor Grand Hotel to find the nuclear key and start Cleansing the Land.
 * 14) Enter the Vim factory and find the secret medical facility to uncover a synth infiltrator in Far Harbor.
 * 15) Find DiMA's stash and recover the killswitch.
 * 16) Return to DiMA and cash in your bounty.