It was going to be a weeklong romantic adventure, rafting like Huck Finn down a fast-flowing tributary of the Amazon.

Instead, it wound up becoming a nearly month long ordeal, stranding two honeymooners on a parasite-ridden lake in the Bolivian jungle — with nothing to eat but slugs and frogs.

Holly FitzGerald, now 71 and living in South Dartmouth, Mass., recalls her riveting story in the memoir “Ruthless River” (Vintage Departures, out now).

“What happened was life-changing,” she told The Post of her hellish journey with her husband. “It gave me a new perspective and shaped the person I am today.”

Subsequent challenges, such as beating breast cancer in 2004, were “less intimidating” in comparison — all because she and her spouse, Fitz, 70, cheated death 44 years ago.

The epic tale began in February 1973, five months into the couple’s yearlong honeymoon backpacking around South America.

There was a general plan to reach Rio de Janeiro and then board a ship bound for Africa, but the newlyweds were frequently sidetracked while meeting interesting people en route. One such encounter, with some anthropologists, aroused their curiosity about the Amazon basin.

So they booked seats on a puddle jumper to the Peruvian frontier town of Puerto Maldonado. It would be their first mistake.

‘I caught sight of the plane’s right wing and engine out of the window. They’d snapped off entirely.’

“The wobbly plane began to descend very fast, pushing me sideways, causing me to grasp the bench,” Holly writes in her book. “We were thrown back and forth, held by our wide seat belts . . . I caught sight of the plane’s right wing and engine out of the window. They’d snapped off entirely.”

The DC-3 — with 13 people onboard — crash-landed in the jungle. Incredibly, no one was badly hurt in the accident, which was likely a result of pilot error. The survivors were escorted across a river to the nearest shelter: an open penal colony full of convicted murderers and rapists.

“Deep in the dense and unforgiving thicket, prison walls were not necessary,” Holly writes.

“The inmates . . . could never escape. It dawned on me that now we were imprisoned, too.”

Although the passengers slept in separate barracks from the inmates, there was a sense of menace in the air. Despite daily promises from prison guards that help was on the way, it was four long days before a plane appeared on the muddy pasture that served as a runway.

When they finally reached Puerto Maldonado, the FitzGeralds discovered they’d missed their boat to Bolivia. It was flood season, and the next available trip downriver was likely three months away.

Encouraged by locals, the hardy young couple decided to build a raft — using four logs and a makeshift tent fashioned from plastic sheeting lined with mosquito netting — and navigate 500 miles of the Madre de Dios river to Riberalta, Bolivia, themselves.

“At first, it was idyllic,” recalled Holly, who reveled in the jungle scents of ripe mangoes and gardenia. “You could read. Relax. Take photographs. We were as free as butterflies, soaking up the scenery and the tranquility because nobody else was on the river.”

That should have set off alarm bells. But the duo took comfort in a local’s prior assurances that, if their raft accidentally took a detour along a tributary, it would always eventually return to the main river.

“He convinced us it would be a straightforward journey,” said Holly of the man. They had nevertheless been warned not to swim in the water for fear of flesh-eating piranhas and candiru fish, which can enter the body through the nether regions and feed off intestines.

On their fourth night, fortunes changed. While the couple was sleeping, a raging thunderstorm brewed. Torrential rain pelted the tent, threatening its collapse.

“Just then, something slammed the bow, pulling the raft downwards,” Holly writes, She heard the horror-movie rip of the plastic tent as a large tree trunk crashed through, pinning her to the raft.

Her husband struggled to pull the tree off her as the small vessel rocked back and forth, threatening to capsize at any second.

“The roots scraped my naked back. They felt like they were going deep,” Holly wrote of the harrowing episode.

Fitz managed to free her, but not before an army of fire ants emerged from the bark and began biting Holly all over her body.

Once the storm quieted, the sun soon came up to reveal a frightening reality: The couple were now off course, with no idea of their location.

“We didn’t know it at the time, but we were stranded in the middle of a swamp — a seasonal lake formed because of flooding,” said Holly. “Most of our food and supplies had fallen overboard during the night. Our tent was ripped to shreds, so we had to replace it with spare plastic sheeting we’d managed to hold onto.”

At first, they thought they had drifted into a tributary of the river and that, if they risked the piranhas and invasive fish, they could swim against a current that was visible in the distance and return to the spot where they’d been blown off course. But once the storm waters settled, their raft was too wide to safely navigate the swamp’s channels, so they had no choice but to swim to try to get help.

As the land around them was submerged, there was no question of getting anywhere on foot. Holly could only hope that what the locals had told them about caimans — a carnivorous, alligator-like reptile — remaining dormant during the rainy season was true.

Tying the raft and their few remaining possessions to a bush above the water line, the duo swam for hours at a time — only to travel less than half a mile. They gave up after trying for two days.

For 26 days, they were marooned — knowing no one was looking for them, as they had written to their family that they’d be exploring for at least a month.

THE couple were besieged by bees, mosquitoes and other biting insects. They tried to catch fish but had zero luck. Strange noises from the jungle terrified them at night, and they felt themselves weaken by the hour. Holly and Fitz became skeletal and frequently doubled over in pain because of the lack of food. One morning, Holly initially couldn’t wake her husband and feared he had died in his sleep.

They whiled away the time by gazing at a water-damaged photo of their beloved dog, Zelda.

“It was a reminder of what awaited us if we could only survive,” Holly writes.

Then, for the first time, Holly became overwhelmed by a desire to have a baby in the future. “Despite my physical debilitation, my mind had achieved a heightened clarity,” she writes. “As starvation consumed my body, its effects trimmed the fat and gristle from my thoughts . . . I could see my place in the grand scheme — I would be a mother.”

This strengthened the pair’s resolve, and they started to comb the trees and bushes standing in the water for frogs, grasshoppers, snails and slugs to eat.

“We had to get over the gag reflex,” Holly recalled. Increasingly desperate, they even consumed potentially poisonous berries, which mercifully turned out to be safe.

On their 26th day in the swamp, Holly had a premonition. “We’re going to make it out of here. I just know it,” she told her ailing husband, who lost 40 pounds during the ordeal. (Holly herself shed 20 pounds.)

Hours later, after nearly a month of not seeing a single soul, they spotted two Indians hunting turtles from a canoe. Using made-up hand signs and broken Spanish, the honeymooners convinced the men to take them to their village. They later traveled to their original destination of Riberalta.

Over the next two weeks, the FitzGeralds were treated in a hospital for exhaustion, severe malnutrition, and the bites and stings they’d received. In April 1973, they finally returned home.

Three years later, their daughter Megan was born, followed by another girl, Aiden, in 1979.

Fitz went on to a career in journalism and worked as a press secretary for Michael Dukakis. Holly was a social worker until she retired in 2012. They now have four grandchildren.

Remarkably, their brush with death didn’t curb the couple’s enthusiasm for travel. They have since visited far-flung destinations including Bali, Malaysia, Ethiopia, Sudan and the Himalayas.

“With hindsight, it does seem rather foolhardy to have taken the raft on such a big river, but I can’t say I have deep regrets,” said Holly. “The whole experience brought Fitz and I closer together.

“Two things I learned in the jungle: Don’t give up, because there is always hope — and use the time you’re given as best you can.”