The Sound has three water entrances, each with its own personality. The main one is the marine entrance from the south, where the English Channel opens into the great bowl of sheltered water behind the Plymouth Breakwater. To the northwest, the River Tamar empties through the Hamoaze, the long tidal channel that hosts Devonport Dockyard, the largest naval base in western Europe. To the northeast, the River Plym disgorges into its narrow estuary, the Cattewater harbour, threading between Mount Batten and the Royal Citadel. The Sound's southwest corner is at Penlee Point in Cornwall, the southeast at Wembury Point in Devon, with Plymouth Hoe forming its northern boundary. Drake's Island, four hundred metres long and a hundred wide, sits in the centre of the northern half. A shallow reef called the Bridge links Drake's Island to the Cornish mainland, less than a metre deep at low water; in the First World War it was reinforced with extra obstructions to keep submarines out of the naval base.

In June 1774, a Plymouth carpenter named John Day climbed into a wooden diving chamber he had built and attached to the sloop Maria. His plan was to descend to the seabed north of Drake's Island, remain there for an extended period, and surface to claim the prize money he had wagered with backers on his survival. He had successfully tested a smaller chamber earlier in the year. This time, with weights designed to be released from inside, he reached the bottom and never came back up. Divers searched for him without success. Day became the first recorded fatality from submarine operations in human history, dying in Plymouth Sound at the very moment that an entirely new category of human endeavour was being invented. The sloop with the chamber still attached lies somewhere in the silt north of Drake's Island; the prize money was never collected; and the engineering questions Day was trying to answer would take another century to resolve.

On 15 July 1815, less than a month after Waterloo, Napoleon Bonaparte surrendered to Captain Frederick Maitland of HMS Bellerophon off Rochefort. He was brought to Plymouth Sound and held aboard the ship for nine days, from 26 July to 4 August, while the British government decided what to do with him. The Sound filled with small boats. Thousands of Plymothians paid for passage out from the Hoe and the Barbican to see the captive emperor with their own eyes, sometimes catching a glimpse of him at a porthole, sometimes seeing him pace the deck. London newspapers sent artists. A genre of paintings emerged, several of which now hang in the National Maritime Museum at Greenwich; John James Chalon's oil canvas Scene in Plymouth Sound in August 1815 is the most famous. Then on 4 August the Bellerophon was ordered to sea; Napoleon was transferred to HMS Northumberland on 7 August at Torbay, and taken to St Helena, where he would die six years later. He never saw France again.

For four centuries the Sound was the most heavily fortified body of water in Britain. The line of defences ran around its entire perimeter: Drake's Island and the Royal Citadel on the inner ring; Picklecombe Fort, Cawsand Fort, Fort Bovisand, Staddon Fort, and Stamford Fort on the outer; and at the very mouth of the Sound, the Breakwater Fort planted in the middle of the great wall itself. Many were built or rebuilt during the Palmerstonian panic of the 1860s, when British anxiety about French rearmament prompted the most expensive defence programme of the Victorian era. They were never used in anger against the threat they were built to deter. They were used during the world wars for anti-aircraft duty and signal stations and observation posts, and one by one they were released by the military through the twentieth century. The breakwater was supervised by Joseph Whidbey from Bovisand Lodge, a house with an uninterrupted view down the full length of his great work.

In January 2019, Plymouth City Council launched a campaign to make Plymouth Sound Britain's first National Marine Park. The proposal has been gathering momentum ever since. The Sound supports kelp forests, seagrass meadows, reefs, and a rich population of fish, seals, dolphins, and seabirds, much of it within sight of the city. It is also working water: the Royal Navy operates from Devonport, the ferries to France and Spain depart from Millbay Docks, fishing trawlers come and go from Sutton Harbour, marinas at Sutton, Mount Wise, and Turnchapel host hundreds of yachts. The King's Harbour Master controls all waterborne traffic. The challenge of marrying conservation to centuries of intense maritime use is exactly the kind of thing a national marine park is designed to address. The Sound has been many things: highway, sanctuary, battleground, departure point. Its next role may be as a place where humans learn to live more lightly on the sea that has always shaped them.