We all need heroes, but the nature of heroism is constantly being redefined.

Each culture and time period has its own slightly different understanding of the idea, and the virtues that a hero embodies change accordingly.

The 19th and early 20th centuries were the last great ages of discovery. Explorers and seafaring adventurers still captured the popular imagination. They were the celebrities of their era, and their exploits became instant myths.

The Endurance: Shackleton's Legendary Antarctic Adventure, based on Caroline Alexander's book, is a feature-length documentary about the last great voyage made in this spirit. On Dec. 14, 1911, Norwegian Roald Amundsen became the first explorer to reach the South Pole. His party beat out by five weeks a British team lead by Robert Scott, who later perished during the trek back to civilization.

Sir Ernest Shackleton had been part of one of Scott's previous, unsuccessful expeditions. But he was determined to be the first to traverse the frozen continent from sea to sea on foot, a feat Amundsen had not accomplished. Director George Butler (Pumping Iron) carefully chronicles Shackleton's failed adventure, which the explorer transformed from a potential tragedy into a triumph of the human spirit.

It was 1914, just before the outbreak of World War I, and most of the British public still lionized explorers in much the same way Americans celebrated astronauts in the 1960s and ‘70s. Five thousand would-be adventurers responded to Shackleton's newspaper ad: “Men wanted for harsh journey. Small wages. Bitter cold.”

The explorer chose 27 of them and, on Aug. 1, they left England for Antarctica. Their ship was named after the Shackleton family motto: “By endurance we conquer.” These sentiments were to prove prophetic.

Not everyone was enthusiastic. Winston Churchill, then Lord of the Admiralty, asserted that “enough life and money has been spent on this sterile quest.” But Shackleton hoped to make some scientific discoveries of importance.

On Dec. 5 the ship left the whaling port of South Georgia Island, the last settlement before the wilderness. In January the temperature dropped from 20 to 70 below zero, causing the polar ice to close in on the vessel. It became trapped there less than a day away from Antarctica's shore. “What the ice gets,” Shackleton wrote in his journal, “the ice keeps.”

The ship was stuck in that frozen vise for 10 months. Shackleton soon realized that the expedition was unlikely to achieve its intended objectives. Rather than go for broke, he showed himself to be a person of great moral character and decided that his primary mission was now to bring everyone back alive. Dealing with the elements was not the hardest job, he wrote. “Dealing with the human spirit is very difficult.”

Shackleton knew how to motivate and control the men under his command. He kept them busy playing soccer, staging plays and racing their sled dogs.

Nature was not kind. When the ice finally shifted, it crushed the Endurance. They abandoned ship, and it sank.

Shackleton made a series of decisions that appeared to make things worse. There was a brief mutiny that he put down without violence. The crew then laboriously dragged three lifeboats across the ice and rowed through stormy seas to the uninhabited Elephant Island.

Forced to endure a blizzard, some of the men went mad. Shackleton realized that their chances of being rescued before the food and supplies ran out were nil. To get help, he selected a handful of his best men and embarked on a 17-day, 800-mile journey in a 22-foot lifeboat to South Georgia Island. Somehow they overcame whales, primitive navigation equipment and a hurricane to reach their destination.

Their troubles weren't over. They had landed on the wrong side of the island, and only Shackleton and two others were healthy enough to undertake the sleepless, 36-hour trek across the mountains to the whalers' station. But these desperate, exhausted men were certain that they were not alone. Shackleton wrote that they sensed a fourth person — Jesus Christ — walked with them to safety.

The others were rescued from Elephant Island. Shackleton had achieved his almost impossible goal. Everyone had survived.

On Sept. 16, 1916, the crew of the Endurance returned to civilization and found that the world had changed. World War I had taken a terrible toll in young British lives, and Shackleton was told he was “the wrong kind of hero.” A more warriorlike set of virtues had understandably come into fashion.

Many of the Endurance's crew volunteered to fight in the trenches. After the war, Shackleton returned to South Georgia Island for a final expedition. There he died of a heart attack on Jan. 5, 1921, and was buried in the whalers' cemetery.

The Endurance's official photographer, Frank Hurley, had recorded the first half of the expedition on motion-picture film and the rest in stills. Butler combines these with present-day footage of the actual locations and discreet reenactments. To propel the narrative, interviews with the survivors' descendants are interspersed with actors' readings of the crews' journals and Liam Neeson's narration.

Shackleton emerges as a figure of quiet courage and perseverance, a person of religious faith who loved physical adventure for what it inspired in the human spirit.

The movie's greatest triumph is to get the audience to view the world through Shackleton's eyes. It's able to understand why he wrote: “We had seen God in His splendors. We had reached the naked soul of man.”

John Prizer writes from Los Angeles.