Seven miles off the English coast sits a rusting metal platform, half the size of a football pitch, perched on two concrete pillars. It has one permanent resident, its own royal family, its own passports and currency, and a history that includes a mercenary coup, pirate radio, and an international money-laundering syndicate. It claims to be a sovereign nation, and it dares the world to say otherwise.

This episode traces the bizarre saga of the Principality of Sealand, from its World War II origins as an anti-aircraft fort to its 1967 takeover by a former army major. Drawing on declassified UK memos and maritime court rulings, we explore how a decommissioned platform became the center of a decades-long geopolitical standoff and what it reveals about how arbitrary the concept of nationhood really is.

• HM Fort Roughs was built in 194 ...