It’s a theatrical moment. A heroic young scientist escapes from a besieged city. At last safely in a hospital, he’s plied with hot soup and medical attention. But it’s too late: He’s too sick to eat, collapsing, dying from the relentless damage already wrought by starvation. When the nurse caring for him undresses the body, she finds a package of seeds, strapped to his chest. Four carefully preserved pounds of grain.

“Why?” the nurse gasps. “He might have saved himself.”

It’s one of the many lethal decisions that populate Simon Parkin’s ethically haunted book, “The Forbidden Garden,” a cinematic telling of the brutal Nazi siege of the Russian city of Leningrad (now called St. Petersburg) during World War II.

It’s also a moral quandary — whom to save? — a dilemma underlined by the book’s subtitle, “The Botanists of Besieged Leningrad and Their Impossible Choice.”

As too often happens, this conundrum was an effect of war: In 1941, as Adolf Hitler’s armies stormed their way across Europe, he and his advisers determined that to also conquer Russia, the city of Leningrad — a cultural, scientific and manufacturing mecca — must fall.

Hitler wanted Leningrad obliterated, “razed to the ground,” as he declared in one tirade. German bombers obliged with one shattering raid after another; ground troops created a heavily armed siege ring around the city, cutting off access to food and medical supplies. The siege of Leningrad lasted for almost 900 days, well into 1944. “By conservative estimates,” Parkin writes, “the siege claimed the lives of three-quarters of a million people, four times the number that died in the atomic bombings of Nagasaki and Hiroshima combined.”

One of Leningrad’s key scientific institutions was the Plant Institute, established in the 1920s by a pioneering botanist, Nicolai Vavilov, as the world’s first great seed bank.

The bank was gloriously international in scope, created from collecting trips around the globe, hailed as “a living museum … unrivaled in completeness by any other collection in the world,” containing a collection that preserved vanishing species and agricultural possibilities for all.

Shortly before the siege began, however, Vavilov’s close relationships with Western scientists led the ever-paranoid Russian leader, Joseph Stalin, to charge him with espionage. Vavilov was tortured into a false confession and imprisoned; he died in prison in 1943.

The scientists at his beloved institute, left leaderless, grappled with how best to preserve their remarkable seed bank amid the destruction of their city and the starvation of their fellow citizens.

Parkin graphically describes a disintegrating city, as food supplies dwindled away to nothing, as people mixed glue, cotton husks, toothpaste powder, machine oil and pigskin belts into soups and stews, eventually becoming so desperate that when their neighbors died, bodies falling like rags into the street, they carved pieces from the corpses for sustenance. “Starvation is a fire that consumes from within,” Parkin writes, describing the way muscles and nerve fibers, brain, heart and lungs dwindle away as the body seeks protein for survival.

And yet, during these years of starvation and suffering, the Plant Institute sheltered some 120 tons of edible seeds. Its scientists hid them, guarded them, refused to distribute them as food even among themselves. They made a pact, all of them, from the young researcher who died still carrying his bundle of seeds, to the botanist who died at his desk, to the unofficial leader of the institute in Vavilov’s absence, a “bean expert” named Nicholai Ivanov, who collapsed one day from lack of food yet refused to dip into the seed stock.

They were desperately determined to save a collection gathered as a bulwark against global famine, even at the cost of their own lives or the suffering of the denizens of the city.

“It was impossible,” said one, “to eat this, your life’s work, the work of lives of your colleagues.” They were afraid to eat even the older seeds, ones that might not germinate, in case they accidentally swallowed one still sparking with life. “So, I propose,” Ivanov said at one bedraggled staff gathering, “that we eat nothing.”

Parkin wrestles almost visibly with this choice throughout the book. In his efforts to establish the decision as a consensus, he enumerates almost every botanist at the Plant Institute, to the point that it’s easy to get lost in a welter of names and specialties.

Still, the overarching question of what is right remains ever-present — a bright, painful line throughout, even when the story occasionally falters. Toward the book’s close, Parkin recounts interviews with modern Russian botanists, seeking to find out if they approve of the choice. They tend to dodge: “It makes our difficulties seem trivial,” one replies.

Indeed.