What is the main conflict in Outlander by Diana Gabaldon?

On a visit to Scotland with her husband, Claire Randall enters the year 1743. After the visit to the standing stones with Frank, she returns alone to retrieve a plant she wanted. Claire so loved herbs and plants. Frank asked her to also look for traces of fire, for it was part of the witch's ritual. When Claire returns, she leans against one of the standing stones and is transported back in time to the brutal and raw Highlands of Scotland.