These four are are a little edgier than previous books. Jack and Annie are on a mission to discover lost stories. The first book is Pompeii as the Volcano is erupting and they are saved by Hercules. Book 14 they go back to Ancient China and the Dragon King who is burning books in the time of the great wall, they are saved by magic silk. Book 15 they save an Irish Monk's story before raiding Vikings and are saved by a sea serpent. The last book they go to the ancient Olympics to save a women's writing and are saved by a Pegasus. I know I shouldn't be annoyed by the mythical saviors in a series about a magic treehouse, but it feels like this is where the series takes an odd turn. Still a cool way to learn about the ancient world.