A girl named Hetty was born in Tibet but now lives in London with her parents. She is one of few people who are known as 'marked' ones, which means that she can see and hear spirits. A deceased marmot who talks, named Minky, is tasked with watching Hetty, since it is now his job after being eaten by a bird of prey. Hetty has a medallion passed that was given to her by her great grandmother, and it turns out that this medallion is able to transport her to her homeland of Tibet.  There she meets a Yeti, who saves her life, even though she is very afraid of the beast.

The Yeti takes her to an ice cavern where Hetty meets her great grandmother, who has now become a ghost. She is the one who gave Minky the orders to go check up on Hetty in London. The old woman, Shappy, is now Hetty’s spirit guide. Hetty is then tasked with being the new caretaker of the Yeti, whose mother was killed by Hetty's ancestors. She is at first reluctant to do this job, but grows attached to the Yeti as time goes on.

Hetty returns to London the next day, but finds herself in a lot of trouble when the Yeti follows her there! The people of the city may harm the Yeti, just as people before killed its mother. It is Hetty’s job to sort this mess out, even though she may not want to, because it is her destiny.