In the expansion, the player character is hired as a private investigator to search for a missing girl in the isolated seaside community of Far Harbor.

The game can be played in first-person or third-person perspective; in either case, the player controls the protagonist throughout their investigation on The Island, a landmass off the coast of Maine.

The puzzles feature a variety of different game mechanics; some require the player to hit targets with lasers, and others allow building using blocks.

[7] The expansion is set on a radioactive, fog-smothered island,[5][6] and is located in Fallout's version of Bar Harbor, a town in Hancock County, Maine.

[6][12] Upon completion of quests, the assisted factions reward the protagonist with Nuka-Cola bottle caps, one of the fictional currencies found throughout the Fallout series.

[14] Some of the quests include investigating minor mysteries, retrieving missing items, solving disagreements, and clearing out monster-infested areas.

[9][18] There are three major conflicting factions present in the expansion, all residing in separate areas: the Harbormen of Far Harbor; the synth (synthetic humans) colony of Acadia; and the Church of the Children of Atom.

The Harbormen of Far Harbor are led by the town leader, Captain Avery, and seek to reclaim The Island from the fog that has gradually driven them out of their homes.

[9][19][20] Valentine's Detective Agency receives a request for help from Kenji and Rei Nakano, a husband and wife living in a remote corner of the Commonwealth:[17] their daughter, Kasumi, has vanished without a trace or explanation.

Arriving in the town of Far Harbor, the Sole Survivor finds The Island locked in a tense stalemate between the local residents and the Children of Atom.

At Kasumi's behest, the Sole Survivor switches focus to investigating DiMA, and gradually learns he has consciously chosen to store some of his memories on hard drives outside of his body.

These are the access codes to a nuclear warhead, stored within the Nucleus, and the means to sabotage the fog condensers protecting Far Harbor.

The Sole Survivor also discovers that DiMA murdered Captain Avery and replaced her with a synth to maintain peace between Far Harbor and Acadia.

[21] The Sole Survivor is able to establish a more permanent peace between all parties by assassinating or chasing away High Confessor Tektus, and allowing DiMA to replace him with a synth who will adopt a more moderate stance towards the Harbormen.

The Railroad, a group opposed to the existence of The Institute with the aim of freeing sentient synths, will send an operative to make contact with Acadia, though the latter will reject their help.

Far Harbor was developed by Bethesda Game Studios and was announced three months after the official release of Fallout 4—alongside Automatron, Wasteland Workshop, and teasers of other upcoming expansions—in a blog post on February 16, 2016.

In particular, Dan Stapleton (IGN) expressed admiration for the new quests but said that the puzzle rooms were "so hard to fail I'm not sure why Bethesda bothered".

[6] Peter Brown (GameSpot) commended the addition of "hours of side quests driven by curious characters",[38] and the staff at Game Central agreed.

[41] Matt Wittaker (Hardcore Gamer) appreciated the quests: he praised the main questline for its focus on "moral ambiguity and tough choices", saying that that was what Fallout fans were seeking.

[13][40] Reviewers were also divided over the storyline, with the writers for Game Revolution admiring the story and new characters, while Peter Brown found them uninteresting.

[5] Chad Sapieha (Post Arcade) said that he was growing weary of the repetitive small tasks like managing loot, and traveling between settlements just to dispose of it; he added that he was done with Fallout 4 and its DLC and was prepared to move onto a further installment.

[49] Comparing the plot of Autumn Leaves and one of Far Harbor's quests called Brain Dead, Veer described both as having “a colorful cast of eccentric robots, in charge of a forgotten Vault where a strange murder happened".

[51] Other similarities were discussed in an article by Mat Paget (GameSpot), such as the fact that both Far Harbor and Autumn Leaves feature the ability to use the character's voice to determine whether they are a robot.