Walk in the Park

Talk to Captain Avery to learn more about Far Harbor. During the conversation, the town will be under attack by the creatures from the fog. Follow Avery then get ready to take out the mutated beasts that will attack the town and the survivors. You can go down and face them head on or you can just stay in the safety of the higher ground. Keep killing the creatures until the attack stops. After defeating the creatures, talk to Avery again and start asking questions. She'll give you a tip that Kasumi may be heading to the synth refuge of Acadia and you need a guide named Old Longfellow to reach the place. During the conversation, she will also ask you to help the town denizens with their specific needs. (Opens a new quest "Living on the Edge")

When mutated salamanders attack

Allen runs the weapon shop in town. If you manage to pass a speech check to make him show some gratitude for helping you, you'll get 150 caps. He's also selling two Legendary weapons, a harpoon gun called Admiral's Friend and The Fish Catcher.

The Last Plank is the bar where you can find Old Longfellow. Pick up the Islander's Almanac on the table beside him. This will unlock the point of interest markers across the island. Talk to Old Longfellow until you get to the point where you can attempt to bribe him or talk your way through in convincing him to guide you. Make the necessary preparations like he suggested then head out when ready.

Follow Longfellow as he leads you to Arcadia. Follow him closely and expect to encounter dangerous creatures along the road. Continue along the way until you encounter an Atom cultist. Continue forth until you reach Acadia.

Talk to Longfellow and he’ll be available as a companion. His cabin, near Far Harbor, can also be used as a settlement. Since this could be your first settlement, you can move some of your settlers to the cabin then send one as a provisioner back to your main settlement (Most likely Sanctuary Hills) so you can immediately establish the settlement conveniently and quickly.