It was inspired by real events aboard QE2 in May 1972 when Royal Marines and Special Boat Service personnel parachuted onto the ship because of a bomb hoax.

The ocean liner SS Britannic is voyaging through the North Atlantic with 1200 passengers on board when the shipping line's owner Nicholas Porter in London receives a call from someone with an Irish accent styling himself as "Juggernaut", who claims to have placed high explosives aboard which are timed to explode and sink the ship at dawn on the following day.

The drums are booby-trapped in various ways, and he warns that any attempt to move them will result in detonation, and offers that technical instructions in how to render the bombs safe will be given in exchange for a ransom of £500,000.

Anthony Fallon, leading a bomb-disposal unit, is dispatched, arriving on the scene by air transit and parachuting, to board the ship and defuse the barrel-bombs before the deadline.

McCleod, whose wife and two children happen to be holidaying on board the ship, leads Scotland Yard's investigation to capture the criminal master-bomber.

After an attempt to drill a hole into a barrel-bomb fails, setting it off and damaging the ship, Fallon decides to split up his team with each man working simultaneously on each of the remaining devices around the ship, Fallon going first with each stage of the defusing operation and coordinating his men by radio link, with the aim that if he fails and his bomb explodes, his men will know what went wrong and continue the process onwards, with his second in command taking up the lead.

However, it contains a hidden mechanism, which his second in command, close friend Charlie Braddock, accidentally triggers, resulting in his death when it explodes, causing further damage to the ship.

Meanwhile, a police search back in London captures the bomber posing as Juggernaut, who is revealed to be an embittered former British military bomb-disposal officer, Sidney Buckland.

By this time Fallon has worked out the important details of his procedure but has no way of knowing which of two options (cutting a red or blue wire) will disable the bombs, and if he chooses the wrong one it will detonate them.

Richard Alan Simmons' script was inspired by a real life bomb threat against the Queen Elizabeth 2 in 1972, which resulted in Special forces (one SAS, two from the Special Boat Squadron and a Welsh bomb disposal expert of the Royal Army Ordnance Corps)[3] being parachuted into the Atlantic to board and search the liner, as dramatized in the film.

In November 1973 it was reported that Bryan Forbes was to direct, with Simmons producing and Richard Harris starring, with filming set to begin in January 1974.

[6] Picker then turned to Richard Lester, with whom he had made a number of films at United Artists, notably A Hard Day's Night.

Lester was finishing work on the Musketeers films in Spain when he got a call from Denis O'Dell saying "We just fired our second director and I've got a Russian ship and we've got to leave on 18 February.

A room at St Thomas' Hospital in Lambeth overlooking the River Thames doubled as the office of the Managing Director of the shipping line.