From the publisher:
The Amazons' mother (doubtless suffering from exhaustion) has gone off sailing in the North Sea with Captain Flint on a rest cure, but she has allowed her two daughters to stay on the lakeshore with their trusty cook. She's also permitted their two old friends, Dick and Dorothea Callum, to come up for a visit. But when the redoubtable Great Aunt hears of their abandonment, she's horrified and off on the next train.
The Amazons are dismayed; not only will their solo holiday be ruined but now they'll have to hide their two guests in the woods in an abandoned shepherd's cottage (where they'll be forced to live off the land like the ancient Celtic "Picts") while the Blackett sisters will be required to dress up in white pinafores, practice the piano, and recite reams of poetry aloud (therefore, "The Martyrs").