The expansion is set in a post-apocalyptic version of Maine's popular tourist destination, Bar Harbor. The location represents the largest landmass Bethesda has ever made for expansion content.
Here is the official story synopsis:
"In Far Harbor, a new case from Valentine's Detective Agency leads you on a search for a young woman and a secret colony of synths. Travel off the coast of Maine to the mysterious island of Far Harbor, where higher levels of radiation have created a more feral world. Navigate through the growing conflict between the synths, the Children of Atom, and the local townspeople. Will you work towards bringing peace to Far Harbor, and at what cost?"
Far Harbor includes more quests to take on and settlements to explore. There are also "lethal" creatures, some of which you can see in the trailer above. The expansion also adds "higher-level" armor and weapons, though no specifics have been announced yet.
The content sells for $25 and is also included with the $50 Fallout 4 DLC pass. The first Fallout 4 expansion was Automatron, which came out in March. This was followed up by Wasteland Workshop in April.
Bethesda also teased that Fallout 4 fans should "stay tuned" for more information about more add-ons coming later this year. If you bought the Fallout 4 DLC pass, you will also receive those expansions, whatever they turn out to be. One possible venue for their announcement could be Bethesda's E3 briefing in June, though this is not confirmed.
On the more immediate horizon, the console versions of Fallout 4 are getting PC-made mods soon, starting on Xbox One in May with a release for PS4 to come in June.