Having enjoyed her tour of the Scottish countryside, Lady Violet Belmaine is reluctant to return to London. Her escort, Hugh St. Sevier, suggests a detour to the Lakes, where his former army connections have invited him to renew old acquaintances. Violet is happy to enjoy the breathtaking scenery, but all is not right with St. Sevier's friends.

The commanding officer's estate is struggling, several marriages from the old regiment appear to be faltering, and somebody is intent on making a bad situation worse. From slashed portraits to sheep trampling the garden, St. Sevier's former comrades are engaged in some sort of rural war against an unknown foe. Sebastian MacHeath arrives to help solve the mystery, but his presence only makes matters more tense. Violet refuses to leave without putting the puzzle pieces together, no matter how dangerous the process or how shocking the eventual solution.