Jen's friend Jack has to leave the school after one more term, to live in London. Jen resigns from the Hamlet Club to play cricket with Jack. Joy suggests that Jen and Jack be invited to stay at the Hall for the summer. Together they spur Joy on to playing practical jokes.

Jack and Jen go looking for a holy oak tree, as they have found out from Joy that the Hall was once called 'Holyoake House.' They meet Gudrun, one of Selma's Swedish cousins. Gudrun longs to be an actress, but her grandmother doesn't approve.

Gundrun's grandmother sends her to England with her uncle. Gudrun hearing only good things about Joan from Selma, has run away to see her. The Uncle comes to the Abbey and tells the girls that his son Karl wishes to marry Gudrun. Gundrun gets to live at the Hall for a while.

Joy's solicitor has purchased on her behalf the ruby that was sold to help Angus. Jen divulges the secret that the jewels have been hidden, and Jack plagues her to reveal the rest of the information. After arguing about it Joy settles things with Jack. Jack and Jen get lost in some underground tunnels, after climbing down a massive tree. The oak tree may have been planted by Ambrose. ( )