In 1925, at the north-western edge of America, children of an icebound Alaskan town are dying.
The Bering Sea-bordered Nome is staring down an "almost inevitable" diphtheria epidemic.
It needs thousands of anti-toxin units to prevent disaster, and the town is all but cut off in the dead of winter.
There's only one answer, albeit treacherous and inherently risky: sled dogs.
Town cut off, ravaged by disease
What had begun in December 1924 as a young girl's sore throat soon grew to a town under quarantine.
Back then, diphtheria —a contagious bacterial infection —was better known as "the plague among children" due to their fatal susceptibility.
As the virus spreads, schools are shut down, houses with the disease boarded up.
Nome's sole doctor, Curtis Welch, panicking, asks the state for one million anti-toxin units to curb the outbreak.
Alaskan health authorities locate 300,000 units but can only get them as far as the nearest train station — more than 1,080 kilometres away.
The precarious path that remains —blizzard-bound and dangerously cold — is unfit for air travel.
But for Nome, a town where "dogs seemed to rule the streets" as authors Gay and Laney Salisbury put it, there was one more method they could try.
An 'inseparably linked' pair
A dozen mushers and their teams are eventually chosen to relay the serum package to Nome, with the trek divided into short stints.
The exception is scrappy Norwegian Leonhard Seppala, dubbed Alaska's fastest musher.
Unlike the others, he is tasked with the relay's western — and perhaps most precarious — round trip, travelling around the frozen Norton Sound for more than 1,010 kilometres.
At his side, "inseparably linked", is lead dog Togo.
"One does not speak of one without mention of the other," a friend of Seppala once remarked of the pair's tight bond.
Togo had been an "infant prodigy" according to Seppala, and with him at the helm, the Norwegian was quietly confident of the distance.
Still, the teams had 20 days, and no time to lose.
A first, cruel setback
The aptly-named Wild Bill Shannon begins the journey, already breaking the sledding "rules of 40s".
It is below minus 45 degrees Celsius ( -50 degrees Fahrenheit) when he ventures out, facing 82 kilometres of rough terrain.
"Hell," Shannon reportedly told a post office inspector as he set off against better judgement, "if people are dying … let's get started".
The weather delivers its first setback.
Shannon's path had been recently traversed by horses who had punched deep holes in the ground, making mine scapes for the dogs.
A frozen river route sees lead dog Blackie only narrowly avoid a potentially path-ending chasm.
As Shannon makes it to the serum's first drop-off spot, the gravity of this plan sinks in.
He is hypo-thermic, face black with frostbite, and three of his dogs are near death.
Too late, Governor Scott Bone calls to stop Seppala from making the second half.
He has already left.
Emergency team assembles
Without Seppala's knowledge, musher Gunnar Kaasen is told to gather a team from the Norwegian's remaining dogs.
Seppala has laid out lead dog Fox in such an emergency, but Kaasan eyes off another: Balto.
Meanwhile, Nome grows desperate with "nothing left" to stop diphtheria's "ravages".
"All hope is in the dogs and their heroic drivers," journalist ER Hyldahl reported.
The serum passes from musher to musher while Seppala still hasn't heard from authorities.
He is midway to his pick-up spot when he spots a random mushing team in peril.
Cautious of his looming deadline, he marches forward, choosing to ignore them, until he hears: "The serum! I have it here!"
A perilous path over the 'ice factory'
With serum in hand, Seppala, Togo and team now journey over the so-called "ice factory".
The Northern Sound, he later estimated, was — with wind chill — a brutal minus 65C ( -85 degrees Fahrenheit).
A storm also pushes inwards, its wind so deafening he can't hear if ice cracks beneath them.
As darkness descends, Seppala puts his faith in Togo.
And somehow, he succeeds, the steadfast husky leading them through more than 130 kilometres in one day.
Blizzard 'impossible for man or beast'
Seppala had handed the package to Charlie Olson, with only 125 kilometres left.
But safety was far from sight.
As Olsen finishes in 128km/h winds, he calls second-last musher Kaasan to tell him the snow is "impossible for man or beast to face".
His line has gone dead, his warning unheard.
What Kaasan faces is snow so thick he cannot see the trail or Balto. He can only hold on.
Once again, the serum's fate is placed into paws.
At one point, the wind lifts and flips the sled, tossing out the package.
A bare-handed Kaasen digs to find it.
At another, Balto refuses to budge, only for Kaasen to find his feet had broken through river ice — the dog stopping to save the team.
As the team nears the final handover, Kaasan makes a decision which will plague him for years.
He pushes on.
The 'oldest of all motors' saves the day
On February 2, in its early hours, Kaasen enters Nome, staggering off the sled and stumbling towards Balto.
"Damn fine dog," he reportedly praises.
The serum is intact. A path that normally takes 20 days is completed in five.
A few weeks later, Nome comes out of quarantine, with its canine heroes already cemented as stars.
"Science made the antitoxin … but science could not get it there," The New York Sun wrote.
"Other engines might freeze and choke, but that oldest of all motors, the heart, whose fuel is blood and whose spark is courage, never stalls but once."
Balto and his team are bought and toured as a sideshow act before they are later rescued by a Cleveland Zoo.
Kaasan's ending is also somewhat haunted — facing accusations he wanted the Nome glory for himself.
"Nonsense," he once snapped at a reporter.
"I was blind."
While Togo, for his part, never revelled in the same fame, his is a happier ending.
"Inseparably", he stayed by Seppala's side until the fine age of 16.
Today, the Iditarod Trail Sled Dog Race is held every March to honour the men and dogs who saved Nome.
“Afterwards, I thought of the ice and the darkness and the terrible wind and the irony that men could build planes and ships," Seppala wrote in an unfinished memoir.
"But when Nome needed life in little packages of serum, it took the dogs to bring it through.”
