Walk in the Park

Walk in the Park is a main quest $$==Quick walkthrough==

Detailed walkthrough
The boat pulls into Far Harbor, where the player character is greeted by two locals, Captain Avery and Allen Lee. Avery asks what they are doing here; the player character questions her about Kasumi. Avery states that Kasumi did come through the harbor, just as a Harborman, the Mariner, sounds an alarm. Avery asks the player character to help defend the "Hull", the wall protecting Far Harbor from danger, in exchange for answers. Follow Avery to the top of the Hull, where the player character must help the people defend the area from two consecutive attacks by gulpers and anglers.

When the battle is over, Avery will converse with the Sole Survivor and Allen Lee about the Fog surrounding the town, how Allen believes it is the fault of the resident group from the Children of Atom, and the name of the synth colony that Kasumi came for, Acadia. Avery directs the Sole Survivor to Old Longfellow, a resident of Far Harbor that is the best chance for navigating the dense, radioactive Fog that surrounds the island, as well as asking them to help around the harbor, starting the quest Living on the Edge. Find Longfellow in the Last Plank, the local bar.

There, the old guide will say that he's done escorting people into the Fog, though an easy Charisma check will convince him to help. Additionally, wearing a suit of power armor while speaking with him will convince him. He advises the player character stock up on items such as Rad-X, ammunition, stimpaks, etc. When ready, Longfellow will lead the Sole Survivor out of Far Harbor, giving them a piece of mirelurk jerky for the road.

Leading through the dense Fog, Longfellow advises the Sole Survivor on various aspects of the island, as they climb higher. At one point encountering a Child of Atom, the player character is warned that the synths are distrusted by the Children but nothing has come of it as of yet. Longfellow leads the Survivor to Acadia, then offers them the use of his cabin and his services as a companion, ending the quest and starting Where You Belong.

Bugs
Due to Old Longfellow's AI script, if the player character runs on ahead, he can often stay in spots where he is scripted to say something. Because of this, if you are to run ahead and talk to the Child of Atom while he is somewhere else, upon returning to the point with Old Longfellow, he will stay there and not do anything since the event can not happen. Reloading an earlier save is the best solution.