Arriving through to the fog to to coastal town of Far Harbor will start this quest.
Speak to Captain Avery where your conversation will get cut short by warnings of an imminent attack. Help to defend the hull and get your first glimpse of new creatures Gulpers and Anglers.
With the town secure, locate and speak to Avery again. She will direct you to Old Longfellow to serve as a guide to the synth refuge, Acadia, to look for Kasumi. Find him at The Last Plank.
Once you've made your acquaintance with your potential guide, Old Longfellow, convince him to help you with your quest. Pick up an issue of Islander's Almanac on the table next to Old Longfellow which will unlock POI map markers across the island. Speak to Longfellow again when you're ready to head out to Acadia. Follow him as he heads inland. Expect a lot more Gulpers, Mirelurks and Ghouls. You'll also meet Trappers, Raider-like hostiles with great taste in headwear.
You will run into a Child of Atom preacher as you continue following Longfellow. Agree to attempt the ritual they speak of and the Nucleus will be added as to your map.
Speak to Old Longfellow when you arrive at Acadia to unlock him as a companion, the workshop at Longfellow's Cabin and to complete the quest.