The Secret at the Gatehouse — Carolyn Keene

Louise and Jean become enmeshed in another mystery when they chance to meet Mrs. Zerbe, who used to work for Mr. Warrington, one of the owners of Captain Dana's ship. Mrs. Zerbe is devastated that she lost her job at Mr. Warrington's estate. During her conversation with the Danas, Mrs. Zerbe mentions a secret which she has kept for Mr. Warrington.

The girls are intrigued but refrain from prying into Mrs. Zerbe's affairs. Later, they are to recall the conversation when Mr. Warrington tries to discover whether Mrs. Zerbe revealed any personal information to them.

The mystery deepens when Mrs. Zerbe dies of a heart attack, Mr. Warrington's valet, Nat Kaner, tries to steal Mrs. Zerbe's belongings, and Mr. Warrington vanishes. The girls are left with a complex mystery to solve which has an astonishing solution.