A truckload of Afghan soldiers had been blown up, leaving bodies strewn across the road.
There were eight casualties, with injuries ranging from shattered legs to shrapnel and head wounds. The combat medics were first at the scene, treating the wounded, applying dressings and giving pain relief.
Then, as the sun was setting, Capt Simpson, from Banchory near Aberdeen, set to work.
The lights from a tractor were used to illuminate the carnage.
She co-ordinated the casualties, assessing their injuries, and had to decide who to treat first. "It's not something I will forget easily. It was hectic, especially in the dark. I was trying to treat people and direct medics at the same time," she said.
Capt Simpson said the four months she spent working in the busy emergency department of the Royal Infirmary of Edinburgh proved to be a big help.
But she said: "I've never dealt with that number of casualties by myself before. It's very different when you are in a hospital and you are with a team of other doctors and experienced emergency department nurses. I didn't panic so much as think of the 20,000 jobs I needed to do all at once. The difficulty was trying to prioritise what to do."
She decided to spend a large part of her time treating a patient who had severe head, neck and chest injuries.
"I think I made the right decision," she said. "When the helicopter arrived he was still alive, and he survived."