Something is wrong at Puffeycup Cove.

Millie Mollusk notices it first when the tide arrives early and the water suddenly begins moving in a direction it never has before. Old landmarks no longer line up. Familiar route markers seem to have shifted. And the safe passage toward the mysterious Drop Off no longer matches the directions recorded on an old chart.

With her friends Phillip, Suzie, and Bulu McGull, Millie sets out to discover what happened.

Their investigation leads them beneath Shadow Ledge, where they meet Gus, a sharp-eyed sculpin who remembers how the underwater bottom used to look. They also encounter the mysterious Vinnie the Viper Eel, who seems to know more than he is willing to say.

As Millie compares old maps, tide records, underwater landmarks, storm damage, and firsthand observations, one question becomes impossible to ignore:

Did someone change the old route—or did the sea change it for them?

The deeper they investigate, the more they discover that the ocean floor itself may hold the answer.

But the Drop Off has another secret waiting below.

The Secret of the Drop Off is a new Millie Mollusk Mystery filled with underwater adventure, clever clues, friendship, humor, and the excitement of solving a mystery by following the evidence—one clue at a time.

And when Millie finally reaches the edge of what she can prove, she discovers that solving one mystery may only reveal an even bigger one.