
However once outside, Nick started to panic and attacked DiMA as he was still under the effects of the experiments, forcing DiMA to "knock the daylights" out of Nick and then leave him to his fate. DiMA explains that he couldn't bear seeing Nick going through all the cruel experiments in the lab and so, about a century earlier, DiMA made an escape with Nick from the Institute.

Upon reaching Acadia, it is revealed that DiMA is a unique model synth like Nick, and calls him a "brother".

But as tensions rise, Acadia is under pressure from both factions to take a side and help eradicate the other. The leader of Acadia, a synth named DiMA, provides the townspeople with the fog condensers while giving the Children of Atom their own base, a former submarine named The Nucleus for them to live in. Stuck between the two sides is Acadia, which only wants peace and harmony. The reason for the tension is the radiated fog that has been slowly overcoming the island the people of Far Harbor cannot live with the fog and they use machines called fog condensers to create fresh air, an act viewed as sacrilege by the Children of Atom who worship radiation and consider the fog the will of Atom to make the whole island a holy place. Traveling to Far Harbor, the Sole Survivor learns that on the island, there are three factions that have been locked in dispute with each other Far Harbor's citizens, the Church of the Children of Atom, and Acadia, a place specially set up as a synth refuge. A brief investigation at the Nakano residence will tell the player that Kasumi believes herself to be a synth replacement, and that she's decided to leave her parents and travel to a place far north called Far Harbor to be with people like her. Arriving at the office, Ellie Perkins will tell Nick and the Sole Survivor to investigate the case of Kasumi Nakano - the missing daughter of a fisherman living beyond the northeast boundaries of the Commonwealth. The add-on starts off with the quest Far From Home when the player character listens to a radio message from the Valentine Detective Agency after completing the quest Getting a Clue, or when the player goes to the Nakano residence (possibly even before meeting Nick Valentine). This leads to the fighting between the Far Harbor citizens and the Children of Atom, but a greater plot involves a synth, who has ties with Nick Valentine.

There, the Sole Survivor will search for a young woman, Kasumi Nakano, and a colony of synths. The add-on is set on Mount Desert Island, off the coast of Maine.
