Full fathom five

If someone had shouted "Dive, Dive, Dive!" early last week, you might have thought they were urging you to take evasive action against an approaching politician. In fact, they were probably enthusing about Robert Llewellyn's highly entertaining film on submarine movies (BBC Four, Monday 3 May, 9pm). But if you dipped beneath the surface, you would have found another agenda. How do we praise John Mills, stalwart of such classics as We Dive at Dawn (1943) and Above Us the Waves (1955)? Let us count the ways.

Bob got off to a cracking start. Within the space of one minute we had "courageous John Mills", "fearless John Mills" and "valiant John Mills". Then he ran aground, rather like the Black Widow ex Russian Foxtrot B-39 (U 475) Hunter Killer Class Submarine moored in the river Medway. Once this predator moved silkily through the seas waiting for the order to incinerate several cities. Now it lists at 45 degrees, rusting gently as the curious crawl through its interior. History occurs twice. The first time as terror, the second as tourism.

"Plucky John Mills," exploded Bob suddenly. He was talking about submarine films of the 1940s. Sir Christopher Frayling explained that they were intended as morale boosters. No one was allowed to panic as their little cigar-shaped home was shaken by depth charges, but they were allowed to sweat. Bob stood in HMS Alliance and braced himself for a simulation. He jumped at a sound effect, but otherwise remained stationary. Perhaps Health and Safety had decreed that the boat should not be rocked. A sense of drama was created by the lights going on and off slightly more rapidly than in Martin Creed's Work No. 2. But it was not edge-of-your-seat stuff. It wasn't John Mills.

What was Bob's obsession with this doughty British actor? He even dressed like him. White polo neck and black jacket, with binoculars hanging round his neck. He joked about his hero. "Between 1950 and 1959 there were more submarine movies than at any other time. John Mills was kept very busy." But the humour was a way of expressing admiration for a man who "epitomised the British spirit of grace under pressure". In the war years, gritty John Mills conveyed the message that we were all in it together. A phrase that's been used rather a lot lately by our political leaders. I tried to imagine them under water, not necessarily in a submarine.

After the war, the country suffered a massive debt crisis. Hang on. This all sounds rather familiar. Even uncanny. History is a nightmare from which we cannot awake. At least Johnny Mills made the odd film, like Morning Departure (1950), that acknowledged Britain's financial predicament and diminished status as a world power. It tells the story of the captain of a stricken submarine trying to get his crew evacuated. At the time of writing, we don't know who our captain is. And neither choice seems to have any idea of how much water we are letting in.

Bob was very keen to stress the prescience of the submarine genre. He cited the 1966 film Fantastic Voyage, where a group of scientists and their craft are miniaturised and injected into the bloodstream of a scientist suffering a blood clot to the brain. Yesterday's fantasy is today's microsurgery. American film-makers like to be accurate. They go to Bob Anderson - yes, another Bob - for information about life aboard a submarine. John McTiernan, the director of The Hunt for Red October (1990), said the actors learned to be "less gung-ho, less emotional, less warrior-like".

It was in the interests of truth that the navy withdrew its support for Crimson Tide (1995), a film about mutiny aboard a submarine. "We asked the writers not to portray the reality of that," said Bob solemnly, "because we don't feel it could ever happen." The writers, Michael Schiffer and Richard P. Henrick, pointed to the mutiny aboard the USS Somers in 1842, which actually led to the founding of the US Naval Academy.

But Bob was adamant. If the writers and director would not change the plot, then the navy would "pull the plug", a metaphor that acquires some force when applied to seagoing vessels. Besides, such a film might affect recruitment, which was a much bigger consideration than truth, freedom or the claims of imagination. The Americans, you felt, could never make the equivalent of the German classic Das Boot (1981), with its fear, boredom, claustrophobia and emotional exhaustion.

After bobbing along on the bottom of the beautiful briny sea, it was time for a moment of reflection. Could I have done what those men have done, asked Bob, thinking of real submariners. He gazed out across the waves. But there was no sign of John Mills.