Each summer Liza, Bill, and Jed visit their grandparents, and they hear the story of the sketches hung above the mantel. The sketches are clues to a hidden treasure, and no one has been able to figure them out for a century. There is a missing first clue, but when the children stumble upon the second clue, they're on their way. Could it be that on this visit they will solve the secret that has eluded so many for more than a hundred years?.

Who or what is taking the kitchen scraps from the garbage every night? What starts out as a puzzle becomes a mystery. Every night Grandma leaves table scraps in a wrapped plastic bag in the garbage can where, according to plan, the McNellis children pick them up to feed their kittens. But the plan changes when the bag is gone in the morning. The twins, Liza and Bill, and brother Jed decide to investigate the matter. First the children discover Liza's missing red sweater in a part of the woods she hadn't been to before, next they read about runaway children in the newspaper, and then their new puppy is lost. Finally they find a teepee in the woods with what seems to be Indians in warpaint dancing and shooting arrows! The young detectives must put all the clues together and solve the mystery.

"Oh, no!" said Liza. "I won't live in a haunted house." The "haunted house' is the old Blake place, and despite Liza's protests, that's where the Roberts family is going to live. Liza, Bill, and Jed soon realize that something weird is happening in and around their new home. Nearly every morning they find mysterious messages. Strange footprints appear, lights flash, and secret compartments pop open. Is John Blake's ghost responsible? If not, who is?

Summer vacation at Pirate Island! Liza, Bill, and Jed can’t believe their grandparents’ wonderful surprise. The excitement begins even before they reach the island, as Grandpa gives the first clue to the unsolved mystery of a long-lost family “treasure.” Whether it’s their frightening run-ins with Hermit Dan, a wild and scary romp with a swooping bat, or a midnight trip into the woods, tracking the series of weird picture clues leads the children all over the island and into all sorts of adventures.

After a frightening run-in with Hermit Dan earlier in the summer, Liza, Bill, and Jed are determined to stay away from the old man. But the stories the islanders tell about him make them curious. Were Hermit Dan’s ancestors really pirates? Why does he avoid people? What’s in the box he keeps buried in the dunes? Liza, Bill, and Jed stir up plenty of excitement when they set out to discover the secrets of Hermit Dan’s past.

Summer vacation is nearly over, and Liza, Bill, and Jed know it’s almost time to leave their grandparents and the exciting adventures they’ve shared on Pirate Island. Before they head home, the trio is itching to solve the mystery of Cougar Island. But it’s private property, and according to legend, trespassers will be haunted forever by the ghosts of the Cougar family. Ghosts or no ghosts, Liza, Bill, and Jed won’t be satisfied until they’ve done some exploring. And when they do, they make the most amazing discovery! Are those old stories true? Could the island be haunted after all?