How does Bod reunite with Scarlett?

Bod and Scarlett were originally good friends together when growing up. They used to hang out at the graveyard quite a lot, and it's there that they discovered a weird, serpent-like creature called the Sleer, who's been guarding a horde of ancient treasure for thousands of years. Eventually, Bod and Scarlett lost touch when Scarlett's family moved with her to Scotland. But years later, when Bod reached the age of fourteen, Scarlett returned to the neighborhood after her parents divorced. The two old friends were reunited and set off once more on their exciting adventures.
Scarlett has made friends with a historian by the name of Jay Frost, who's going to help Bod find out what happened on that terrible night when his family were murdered. This Mr. Frost seems to know a lot about these tragic events. But as Bod will soon discover, there's a very good reason for that: he is none other than Jack Frost, the very same man who killed his family.