Billie Bradley at Treasure Cover is book six of the Billie Bradley series. This book opens with Billie and her two best friends, Laura Jordan and Violet 'Vi' Farrington, walking in the woods around Lake Molata, where their boarding school, Three Towers Hall, is located. They are discussing a letter from Myra Bossenet (formerly Huldah), the subject of their adventure the last summer from book five, Billie Bradley at Twin Lakes. It's now the following spring. Myra is making progress in her art lessons. The girls were thinking of going back to Twin Lakes, but Vi has learned that the cottages they rented have been sold. They don't want to rent different cottages.

The explosion that ended chapter one leads to the excitement of the girls trying to escape from further explosions. They come across an old man pinned to the ground by the branches of a felled tree. They free him, but he has an injured leg. The girls are very worried that the explosions mean that Boxton Military Academy on the opposite side of the lake has been blown up. Three of the students there are Billie's brother, Chet, Laura's brother, Teddy, and their good friend, Ferd Stowing. Luckily, the academy is fine. It would be nice to know who touched off the explosions, though.

The girls find out that the elderly man, a former sailor named Ben Halcomb, was going to Three Towers Hall to see his cousin, Miss Pauline Halcomb, the new math teacher there. Ben has a treasure map and he wants his cousin to help him. The extremely practical teacher isn't interested, but the girls are.

Before the girls and the boys travel to Treasure Cove to help old Ben, there's the matter of a big tennis match between Billie and Amanda, each with a doubles partner. Amanda did something to hurt Billie's game, but old Ben fixed that. There's also a dance at Boxton combined with a baseball game against Boxton's bitter rival, Claxton Hall.

Teddy and Billie are still interested in each other, but Chet seems to have switched his interest from Vi to Laura since book two, Billie Bradley at Three Towers Hall. Luckily, Ferd, who had seemed interested in Laura in that book, has switched to Vi. ( )