"Neptune" was built at Toulon and launched in 1803. She was commissioned in time to join an abortive attempt to break the British blockade of the port in October 1804, but the fleet was forced back to port by bad weather. She sailed again with the fleet, under Vice-Admiral Pierre-Charles Villeneuve, in early 1805, and this time succeeded in breaking out of the Mediterranean, and sailing to the West Indies, where the fleet was joined by Spanish ships. After achieving little in the Caribbean, the fleet sailed back to Europe, where they were engaged by a British fleet in the Battle of Cape Finisterre. "Neptune" saw little action, and was relatively unscathed. She took part in the final fleet manoeuvres, and ended up blockaded in Cadiz by a British fleet under Lord Nelson.