“You cast your net wide and see what happens. Then I saw this gamekeeper pop up.”

“His profile hit me, and he was in a kilt and said he was a 6ft 4 lonely Highland contractor looking for someone to keep him warm on cold nights on the farm.

“He had a hunting agency, and I liked that sort of idea of a man who, if Armageddon came, could fix anything and hunt anything. A man’s man.”

The pair soon found themselves hiking together and falling head over heels, swept up in a whirlwind romance that quickly escalated into an engagement, much to the shock of their friends and family.

How did Caroline find out about the crime?

After celebrating their engagement with friends, Caroline asked her fiancé if there was anything she needed to know. Sandy grew sombre, and she briefly worried he might have a secret family hidden away somewhere. Instead, he revealed something far darker: years earlier, he and his twin brother, Robert, had hit a cyclist while driving under the influence. Rather than calling for help, they switched vehicles and buried the body.

Caroline was suddenly one of the only people in the world to know the truth. Back home, she began searching missing persons cases and discovered that, around the same time three years ago, Tony Parsons had disappeared during a 104-mile (167km) charity bike ride from Fort William to his hometown.

Caroline was left with an impossible decision.

“Imagine you fell in love with someone who made you feel accepted, wanted, seen, whole and loved and just special,” she explains in the documentary.

“Then they’d say to you they did a horrible, horrible thing, something so vile it flips your entire world upside down.”

“To remain in love, you have to keep the secret even though you know it will destroy you. But if you reveal it, you destroy everything too.”

What happened to Dr Caroline Muirhead?

When shown by Sandy where the body was buried — as he wanted her help moving it — Caroline left an empty Red Bull can at the site to mark the location.

She reported the crime on December 27, and the twins were arrested three days later. During police interviews in Glasgow, both men refused to comment. Acting on Caroline’s information, officers discovered the body at a so-called “kill site,” where hunters leave animal remains.

Despite this, Caroline remained romantically involved with Sandy, even allowing the brothers to stay at her apartment in Glasgow. She continued to gather evidence but felt deeply conflicted about leaving the relationship — a fact that was later used against her.