"Neptune" joined the rest of the Toulon fleet preparing to sail for the West Indies on 21 October 1804 under the command of Vice-Admiral Pierre-Charles Villeneuve. The fleet was to transport 6,500 troops to attack British possessions in the Caribbean, but was unable to leave Toulon owing to the presence of a British fleet under Vice-Admiral Horatio Nelson, that was blockading the port. It was not until early January that Villeneuve judged the situation favourable enough to make a sortie. "Neptune" sailed with the 20-strong fleet on 17 January, but were sighted by the patrolling frigates of Nelson's inshore squadron. Villeneuve struggled on, but bad weather in the Gulf of Lyons dispersed his fleet, and fearing an attack on his disorganised forces, returned to Toulon three days after setting out. Villeneuve had in fact managed to give Nelson the slip. The two patrolling frigates, and , had rushed to report the news to Nelson, anchored at La Maddalena, and in so doing, left the French unobserved. Nelson had rushed his fleet to sea, anticipating that Villeneuve would heading east. Nelson spent nearly six weeks sailing back and forth across the Mediterranean through heavy seas while the French remained in port, before news finally reached him of Villeneuve's location.