Although Rockland Ranch is a remote outpost, the lives of its inhabitants embrace the here and now. Picture: YouTube.com

Catrina Foster has one word to describe the early years of her marriage. "Fairytale," the 37-year-old says with a smile. "We were so completely in love you don’t think it can get any better. But it did."

The reason for her increased happiness may come as a surprise. For while the couple were blessed with children, what really "enhanced" the relationship, as Catrina tells it, was the arrival of another woman.

Five years into their marriage, her husband Enoch took another wife — Lillian, a pretty brunette seven years Catrina’s junior. And he did so at Catrina’s suggestion.

"She just seemed the right fit," Catrina says. "There was something special about her."

For most women, of course, introducing another woman into the marital home would be unthinkable. But not for the residents of Rockland Ranch, a small community in the eastern desert of Utah, in the western United States, whose lives are laid bare in an extraordinary four-part Channel 4 documentary series in the UK.

In Rockland Ranch, most families are based around polygamous — they prefer the term "plural" — marriages, with the majority of the men juggling at least two wives and often an astonishing number of children.

The residents are fundamentalist Mormons who believe polygamy is a way of mirroring God’s love for all people. And surprisingly, given that polygamy usually conjures up images of dowdy, submissive women, these "sister wives" are not only modern-looking and charismatic but insist their unorthodox acceptance of multiple wives has made their marriages stronger.

"Making this choice honestly had a way of deepening my relationship," insists Catrina, a stunning 37-year-old blonde. "When you put energy into something that doesn’t come naturally and stop thinking only about yourself, you can get something beautiful back."

"We’re not these quiet, submissive women," adds Suzy Morrison, a 36-year-old brunette and the first of postman Abel Morrison’s three wives. "We live a normal life — we have mobile phones, we eat pizza, we care about how we look. Yes, we go home to a husband and other wives but we do it by choice, not because we were led into it."

This week, viewers will get a chance to see if they agree. Last year, Suzy, Catrina and others at Rockland Ranch allowed a crew to film them over the course of a year as they went about their daily lives — the first time they have allowed TV cameras in.

The result is a candid, compelling series which Catrina hopes will "blow away some misconceptions" about polygamous unions and perhaps even inspire British people to consider the lifestyle.

Although Rockland Ranch is a remote outpost, the lives of its inhabitants embrace the here and now.

There is a communal swimming pool, trampolines in the garden, and daily life is governed by the usual chaotic timetable of family life, from dentist visits and piano lessons to negotiations over television time.

Catrina and Enoch live in a huge house alongside Enoch’s second wife Lillian, 30, and the 19 children they have between them (Catrina has 11 and Lillian, who is five months pregnant, has eight).

Generally, says Catrina, they "take it in turn" to have children. They also alternate in helping Enoch run his delivery business.

"For the kids, we are all seen as their mothers. We don’t differentiate," says Catrina. "One mom works while the other stays at home, depending on who’s just had the baby. It works really well."

Enoch also alternates the nights he spends with his wives.

The idea of your husband living married life to the full with another woman in another part of your house would be hard for most wives to stomach. Not Catrina.

While she wasn’t raised in a plural marriage — or even as a Mormon — after marrying childhood sweetheart Enoch at the tender age of 17 and moving to Rockland Ranch, she says she found herself increasingly attracted to the notion.

"It honestly wasn’t discussed at first, even though Enoch came from a plural marriage," she recalls. "His perspective was that he was open to the idea but it had to be something I chose. And as the years went by I felt myself opening up to it.

"It wasn’t even so much a religious thing as something in my heart. I remember thinking there’s not many amazing men out there and I wanted someone else to be able to share this man who had so much love to give. And I wanted to be part of something bigger."

So 12 years ago, when she was 25 and already a mother of four, Catrina suggested Lillian, a family friend, might become wife number two.

"She and her brothers came down for a camping weekend and I just felt a connection to her," Catrina recalls. "That weekend I approached Enoch and said: 'There is something out of the ordinary about this girl.' And we took it from there."

For her own part, Lillian, who was then only 18, admits she was ready for a different sort of life. "I came from a very party lifestyle. I started drinking when I was 11. But by 14 I already knew I wasn’t happy and that I was looking for another way to live. And when Catrina and Enoch approached me I just felt that was where I wanted to be, where I wanted to start my family," she says.

Aside from holding hands, courtship must remain respectfully chaste, and Lillian admits it was "a challenge" coming into an established family dynamic once she and Enoch had exchanged vows. "It helped that Trine [Catrina] always treated me like a friend," she says.

Catrina concedes that while there were "jealous moments", they were fleeting. "It helped to focus on the bigger picture. And actually, working on getting rid of these selfish thoughts in your mind is a pretty amazing experience."

Amazing, perhaps, but not always easy, as Lillian will now testify. For as the documentary was being filmed, Enoch was wooing a third prospective wife in the shapely form of 25-year-old nanny Lydia.

While the arrival of a new wife must be a collective decision — meaning all wives and children must agree on welcoming her into the family — Lillian admits she has struggled watching her husband fall in love with another woman, particularly one who is, in Lillian’s words, "young and beautiful" and with a figure unspoilt by childbirth.

"Someone can tell you all day long that having a baby without an epidural is painful, but until you experience it for the first time you can’t really know how it feels. And that’s how it felt for me with Lydia," says Lillian.

"I’m not going to lie, there are times it really hurts, but that’s one of the reasons I chose it. It gives me the opportunity to grow past my own little demons to become a better person. Can I look past myself and what I want and want that for somebody else?"

It is a sentiment shared by Suzy Morrison, mother to six of Abel Morrison’s 12 children, who range in age from three to 15.

Raised within a plural marriage, Suzy grew up convinced that it was something she wanted to replicate for herself. Indeed, she was so convinced, that it was Suzy who raised the idea early in her growing relationship with Abel.

"I knew it was what I wanted. And I didn’t want to have a serious relationship with someone who wouldn’t want to pursue that," she says. "It just felt right to me, the goals and the principles.

"It is doing something for a bigger cause — you have to behave in a way that is beneficial to the whole family in order to be successful."

The couple embraced plural marriage early on. Suzy was 21 when she married, and a year and a half later, having just given birth to her first child, she decided it was time to welcome another wife into the family. So Beth became Mrs Morrison the second.

And, like Catrina, it was Suzy who brokered the deal. "I knew Beth from high school and I knew she had a thing for Abe prior to me dating him, so I took it from there," she recalls.

Beth, although she too was raised in a plural family, admits becoming a "sister wife" was originally not in her plans.

"I had come out of a relationship with a man I thought I was going to marry and was making plans to move out of state when Abe called to ask me out to dinner," she says. "Well, I guess that was that."

Now mother to four girls aged between four and 12, Beth, 37, agrees it wasn’t easy to undergo her four-month courtship with Abel under Suzy’s scrutiny: "It’s hard to get to know someone when you’re really having to get to know two people.

"There is a lot of us hanging out together as a threesome," agrees Suzy. "Beth found that tough."

On occasion, petty jealousies have also reared their heads.

"We are all capable of that," acknowledges Suzy. "There are times when maybe something special is happening to one of the other girls and you have a jealous moment. But you learn you can’t focus on it."

The arrival three years ago of Marina — wife number three and, at 27, ten years younger than both her predecessors — seems to have proved tricky for both women.

The curvaceous blonde is now the mother of a nine-month-old baby and a two-year-old girl but did not slot easily into the existing family dynamic.

"It was hard," says Beth. "I was pregnant when they were courting and my baby was only a couple of months old when they married, so, of course, you feel a bit replaced. It also made me realise how hard it must have been for Suzy when I came into the relationship."

Marina has another word for it. "It was a disaster," she openly acknowledges. "The first year was hell for all of us."

In Marina’s case, it seems, it was a clear case of reality bites.

"My wedding day was diamonds and rainbows, then after, as life starts setting in, it was like, 'What did I do?' "

She found it tough not least because it was deemed sensible that, as she quickly fell pregnant with her first child, Marina should become –— temporarily at least — stay-at-home mum to all the other Morrison children, allowing Beth to work in a local bank and Suzy to continue training as a nurse.

Often chained to the sink, Marina admits that she has had doubts."One thing I struggle with is believing that Abel loves me as much as he loves them," she says.

Some people, of course, will think the opposite: that set-ups such as these exemplify a man having his cake and eating it. Yet the concept has Catrina hooting with laughter.

"In fact it’s quite the opposite," she says. "I think a lot of people think it’s about the sex — 'Oh, he can have sex with all these women!'. Well, any man could go out and do that anyway without any commitment.

"Trust me, that has very little to do with anything. Can you imagine having to deal with not one but possibly two or three women, with all their complications and emotions? It’s a sacrifice and a huge labour on the men’s part.

"The men in this lifestyle give so much. Enoch is committed to these relationships, to his children and building his family."

For all that, choosing life as a "sister wife" remains a lifestyle many will struggle to understand.

"I know some people think it’s crazy," adds Lillian.

"All I can say is, I’m very content with my life. Ultimately, I would much rather have a piece of a really good man than a whole man who wasn’t as good."