'Lucid' was a British attempt, planned as 'Lucifer', to destroy the barge forces which the Germans were concentrating in Dutch, Belgian and northern French ports, rivers and canals for use in the planned 'Seelöwe' invasion of the UK (7 October 1940).

After reconnaissance and intelligence reports had confirmed that the Germans had started to concentrate barges suitable for invasion use, the Royal Air Force was ordered to make bombing attacks on these, but none of the raids was more than marginally successful. A series of experiments was then undertaken by the Petroleum Warfare Department to establish whether or not barge concentrations could be attacked and destroyed before they could reach the British coast and disgorge the men and equipment they were carrying. The first idea was to use a vessel filled with petrol, which could be steered into a barge concentration and there exploded to scatter burning fuel over a large area. The concept was evaluated over the Maplin Sands, where Suffolk, a Thames river tanker filled with 50 tons of petrol, was exploded in shallow water.

Another concept was to use a floating mass of oil contained by a ring of coir matting: assemblies of such rings were to be carried on the sterns of adapted ships and then released onto the surface of the sea. Trials of the concept with a vessel named Ben Hann produced a ribbon of flame almost 1,000 yards (915 m) long and 7 ft (2.1 m) wide, and which could be towed at 4 kt.

However, none of the experiments led to the production of a technically and tactically viable means of attacking barge concentrations at sea. The Suffolk trial did nonetheless provide the basis for a more ambitious concept to set fire to barge concentrations before they left port and, launched in June and July 1940, this was the origin of 'Lucid'. Like many such innovatory, or perhaps fanciful, notions, this gained the enthusiastic support of Prime Minister Winston Churchill, a history loving leader to whom the the idea of using fire ships against the German invasion smacked nicely of the English fireship attack on the Spanish Armada in 1588.

The planning and execution of 'Lucid', to destroy wooden invasion barges at Boulogne and Calais in north-eastern France, was entrusted to Captain Augustus Agar, who selected Lieutenant Morgan Giles to serve on his staff as Giles had considerable experience in setting explosive charges. Tankers of the type needed for the operation were in very short supply at the time, and in combination with the fact that these were to be destroyed during the operation, this led to the selection of old and very small tankers currently used only for immobile storage purposes. Four of these vessels were taken in hand for the necessary modification and restoration to limited seaworthiness so that they could be moved, as deception 'leaks' suggested, for use as blockships.

Another problem faced by Agar was the difficulty of obtaining reliable motor boats in which the crews could make their escapes, and it would have defeated the object of the blockship ruse had high-quality motor boats been made available.

Capable of a speed of less than 6 kt and of very poor reliability, the vessels readied for 'Lucid' were the 5,218-ton tanker Oakfield and the decommissioned 5,605-ton Royal Fleet Auxiliary oilers War Nawab and War Nizam. The ships were quickly readied for their task and filled with a PWD-developed cocktail of 50% heavy oil, 25% Diesel oil and 25% petrol. Each of the tankers carried between 2,000 and 3,000 tons of this blend. To this basic load was added an explosive charge comprising cordite, nitrocellulose and old anti-submarine bombs to ignite the petroleum cocktail and help hurl the burning mixture far and wide.

The plan demanded ideal conditions of wind and tide as the three tankers made a night passage to the entrances of the target ports. Here each tanker would be abandoned by all but two or three of its crew, the remaining men setting the timers for the fuses and aiming the vessel right into the harbour before escaping at the last moment by motor boat. As the tankers exploded in or close to the harbour entrance, it was left for the rising tide to carry the burning fuel mix into the harbour and engulf in flame all the vessels there.

Late in the afternoon of 26 September, War Nizam and Oakfield departed Sheerness in Kent bound for Calais, and War Nawab departed Portsmouth in Hampshire bound for Boulogne, and at much the same time the last preparations were being made for a diversionary bombing attack on Ostend. A number of destroyers, motor torpedo boats and other vessels escorted the three tankers, and the whole operation was controlled by Agar from the destroyer Campbell.

The wind soon become unfavourable, however, and mechanical problems then caused Oakfield to be dropped from the undertaking, with War Nizam falling foul of boiler problems slightly later. With only War Nawab now left, and not wishing to sacrifice the element of surprise, Agar now decided to cancel the operation. The recall order reached War Nawab when the vessel was only 7 miles (11.25 km) from Boulogne.

Another effort was made on 3 October, but this was defeated by bad weather, as was another attempt on the following night. On the night of 7/8 October a fourth attempt failed after the escort destroyer Hambledon, the command ship with Agar on board, was severely damaged by a German mine off the South Foreland in the English Channel and had to be taken in tow for a return to the south coast of England. The ships were shelled by the German coastal batteries on the French coast on their way back, but took no hits.

Another attempt was planned early in November, but as Adolf Hitler had by then clearly postponed 'Seelöwe' the Admiralty decided that there was no short-term purpose in trying to complete the implementation of 'Lucid'. The operation’s concept was revived in the spring of 1941, but not implemented.