An extortionist hides six time bombs on a cruise liner, the SS Britannic, and demands a £500,000 ransom in return for information on the bombs' locations and instructions on how to disarm them without setting off their booby traps. A Navy bomb disposal team, led by Lt Commander Fallon (Harris), is flown out to the ship and attempts to disable the bombs, while on land a team of investigators led by Superintendent McLeod (Hopkins), whose family are on board the Britannic, race against time to uncover the identity of the bomber.
This film contains examples of:
- Alan Smithee: After Richard Lester had the screenplay substantially rewritten by an uncredited Alan Plater, the original screenwriter asked to have his name taken off it, and the on-screen credit is given to the pseudonymous "Richard De Koker".
- Blatant Lies: When the passengers have to Abandon Ship in very bad weather, a member of the crew assures them they'll soon be picked up. An American politician (being a Consummate Liar) tells him he's full of it.
- Evil Sounds Deep: Averted; the villain talks in a somewhat prissy voice when making his demands, to hide his identity.
- Foreshadowing: Early in the film, a member of the bomb disposal unit named Charlie mentions to his boss Fallon that he's passed on an opportunity to take a different job. Later, while both Charlie and Fallon are disarming two separate bombs, Fallon quips, "Haven't I told you about death? It's nature's way of saying you're in the wrong job." Shortly thereafter, Charlie's bomb goes off, killing him.
- Heroic BSoD: Fallon has one.
- One-Word Title
- Phone-Trace Race: This trope plays out when the bomber calls to ask if the ship's owners have decided to pay the ransom yet. They successfully trace the phone that the call is coming from, only to find that it's just a relay, attached earpiece-to-mouthpiece to another phone which is receiving another call from the bomber's actual location.
- Ransom Drop: The bomber specifies a place and time for the ransom to be dropped off; the police stake it out, but the pick-up is made by a hireling who doesn't know who he's working for. The villain's plan fails because this hireling has spent the money to be used for the excess baggage fee.
- Stiff Upper Lip: Averted; the passengers quickly work out the crew are nervous. When they're finally told the truth, the entertainment officer tries to raise their spirits to no avail, finally giving up and admitting that he's just as terrified — a Convenient Slow Dance works a lot better.
- Time Bomb: Six of them hidden in various parts of the ship and set to go off when the extortionist's deadline expires (plus a smaller one set to go off at the same time as the extortionist delivers his demands, as proof that his threat is not a hoax).
- Wire Dilemma: In the days before this was a tired cliché, everything comes down to Richard Harris, a pair of wire cutters, and two wires. The police back in London have captured the bad guy, and he tells them to cut the blue wire — so now the question becomes, do you believe him, or cut the red wire?