Fort Flagler occupies 784 acres at the northern tip of Marrowstone Island, where the land points like a finger into the convergence of Port Townsend Bay and Admiralty Inlet. In the late 1890s, the U.S. government chose this spot as one of three fortifications designed to guard the entrance to Puget Sound. Fort Worden stood near Port Townsend, Fort Casey perched on Whidbey Island, and Fort Flagler completed the triangle from Marrowstone. Military planners called it the Triangle of Fire, a ring of disappearing coastal guns positioned to destroy any hostile fleet attempting to reach Seattle, Tacoma, or the naval yards at Bremerton. No enemy fleet ever tested the arrangement. The guns were eventually removed, and Fort Flagler became a state park where visitors camp among the old concrete bunkers and walk the beaches that were once firing lines.

Marrowstone Island is 6.3 square miles of land separated from Indian Island by the narrow waters of Scow Bay and Kilisut Harbor. Until 2022, a causeway connected the two islands at their southwestern tips. That causeway was removed and replaced with a low bridge to restore salmon passage into Scow Bay, reopening a channel that had existed naturally before human engineering closed it. The island's western edge, along Flagler Road, is low-lying. The eastern side rises into sandy bluffs, with the exception of East Beach, a sandy stretch at the northern end of East Marrowstone Road. Mystery Bay, an inlet on the western side, hosts a small state park primarily for boaters. Kinney Point State Park covers 76 acres at the island's southern end, accessible only by water and part of the Cascadia Marine Trail. Nodule Point juts from the mid-island eastern coast. The geography is a study in small variations: bluff here, beach there, a bay named Mystery for reasons nobody seems to remember.

For roughly a century, Marrowstone has held its annual Strawberry Festival in June. Strawberries were once grown across the island and still grow wild, and each year residents and visitors gather at the Nordland Garden Club Building for strawberry shortcake that tastes like a tradition older than anyone serving it. Every Memorial Day weekend brings the Tractor Days Parade, during which island residents drive their tractors past the Nordland General Store in a procession that is equal parts agricultural heritage and small-town theater. The store itself, damaged by fire in November 2020, was the island's only source of groceries and its unofficial town center. Its 2024 reopening as a community co-op was treated not as a business event but as a civic resurrection.

From 1958 to 1989, Fort Flagler hosted the Seattle Youth Symphony's Marrowstone Music Festival, a summer program originally called the Pacific Northwest Music Camp. For three decades, young musicians filled the old military barracks with orchestral rehearsals, the sound of strings and brass echoing off the bunker walls and drifting across Admiralty Inlet. The festival eventually outgrew the island and moved to Western Washington University in Bellingham, but it kept the Marrowstone name. In another layer of cultural improbability, the neo-pagan Aquarian Tabernacle Church performs a modern version of the ancient Greek Eleusinian Mysteries at Fort Flagler each Easter weekend. On an island where the median age is 60.5 years and the population skews heavily toward retirees, these rituals, both musical and spiritual, hint at a community more eccentric and layered than its demographics might suggest.