Brother detectives Frank and Joe search a hidden castle for clues to help them find a missing comic book in the 19th book in the thrilling Hardy Boys Adventures series.

Frank and Joe are back on Black Bear Mountain, the scene of a previous wilderness adventure and mind-bending mystery. This time, the brothers are checking in on Dr. K, a friend they made during their first trip. Dr. K is a fan of living off the grid, but he’s been MIA long enough to cause worry in the small mountain town. And so the teenage detectives Black Bear Mountain relied on before have been called in to help once again.

One of the Hardys’ favorite writers, Nathan Foxwood, has recently died in a tragic car accident. Now, the press is swarming his house in Bayport to get the scoop on the novel he completed just before his untimely death. When Joe hears that Nathan’s wife is having a giant estate sale, he drags Frank with him. Who could pass up the opportunity to see inside their favorite author’s home? Nathan’s wife says she wants to get away as quickly as possible. She’s sure the house is haunted. But Nathan’s assistant, Adam, is not so willing to blame it all on the supernatural.

Frank and Joe Hardy have been invited to take part in a conservation mission by a marine biologist at the Bayport Aquarium. The boys get to go on an all-expenses-paid trip to the lush and beautiful Florida Keys to help save sharks! But the trip’s purpose is suddenly sidelined when a paddle boarder goes missing, and rumors start that the disappearance is the result of a shark attack. After spending so much time learning about the local shark population, the Hardy Boys aren’t buying the story.

An original Sherlock Holmes manuscript was stolen from the Bayport Museum and now pages keep showing up in Frank and Joe's things: their lockers, menus, even the backseat of their car. It's clear someone is out to frame them. But who could it be? The boys try their usual means of deduction, but everything leads to a dead end. They just keep finding unsuspecting people who were paid to hide the pages by a mysterious third party. Frank recognizes this tactic - it's exactly what Moriarty would do to throw off Sherlock Holmes. So the brothers decide to use one of Sherlock's signature ideas.

"JOE!" Frank suddenly yelled behind me, but it was too late. A dark shape advanced on us from inside the doorway, and a bag was thrown over my head again. I could hear them doing the same to Frank too. Arms reached out and roughly pulled me inside....

Something is not right in Bayport. After a series of unfortunate (yet harmless) pranks, it looks like Frank and Joe have caught their suspect surprisingly early. Their classmate, Seth Diller, has taken his love of filmmaking a bit too far. But shortly after Frank and Joe confront Seth, there are fresh outbreaks of Seth-like pranks. Except these aren't as harmless. After a bit of investigating, the Hardy Boys uncover evidence of the Red Arrow - a group that will stop at nothing to bring chaos and crime to Bayport's peaceful existence.

"Joe," I said, concerned, "hit the brake!" "I am," Joe said, pumping the pedal now. But the car didn't slow at all. The traffic light turned red as we hurtled toward the intersection. And suddenly I saw the one other car out this early on a Saturday morning - a huge Express Delivery truck! Joe screamed. I leaned forward, bracing myself against the dashboard. There was nothing we could do. It was speeding right at us!

Bayport High's Green Environment Conservation Club is camping on Black Bear Mountain. Not only does the mountain live up to its name - the fierce, furry animals are everywhere - but rumor has it that a madman lurks in the nearby forest, and the crazy dude has a penchant for feasting on human flesh. When the campers wake up, however, not only do they find a bear roaming the campsite, but their teacher is missing...and blood is streaked across his tent.

When a sinkhole opens up and swallows one of Bayport's historic statues, Frank and Joe suspect something - or someone - shady is at play. They jump at the chance to investigate: Joe as a member of an urban exploration club and Frank as a photographer for the school paper. But in the thick of their search, the boys stumble upon an entire underground city that's been abandoned for years. Now the city is home to a cultish band of criminals who plan to take over Bayport.

As I rounded the corner, I spotted Rick just ahead. He paused and bent down to poke at something on the ground by his feet, muttering something that I couldn't make out due to the wind. WHOOSH! I gasped and jumped back as a huge pile of snow slid off the roof of the ski shack, completely burying Rick!

A wave of motion rippled through the cattle as they broke into a run. The cows directly behind Sarah and me looked back anxiously. The entire herd would be upon us in no time. Sarah cupped a hand to her mouth. "Clear out of the way! Stampede!"

The Hardy Boys are visiting Paris! Joe is excited to check out all the street art he's heard about. And Frank can't wait to go to all the museums he's researched. On their very first day in the city, the brothers are woken up early in the morning by fireworks. But these are no ordinary fireworks. They are a part of elaborate cover for the heist of a priceless artifact.

A twig cracked. I snapped my head to the side, following the direction of the sound, and peered through the greenery. There was nothing there. As I began to turn back around, I caught the slightest motion out of the corner of my eye. Peeking out between clumps of dense vegetation, about 20 feet away, were two eyes. My skin prickled when I realized that they were staring back at me.

The world's largest reef exhibit is coming to the Bayport Aquarium. Frank and Joe are stoked for the unveiling of Predator Reef, a 300,000-gallon tank that houses small sharks, rays, and thousands of colorful fish. It's also home to the aquarium's new mascot, Captain Hook - a rare and valuable, 500-pound, giant sea turtle.