A summer trip back to Ireland with their neighbor and friend, Mr. Amos Murphy—a retired Irish detective—ushers the Sullivan Twins into a thrilling adventure that they will remember for years to come.

The boys, eager for excitement, are once again joined by the high-spirited Irish siblings, Ciara and Finley O'Brien. Together, this dynamic team joins forces to unravel the captivating mystery of The Dullahan, a legendary figure steeped in Irish folklore, who is said to make an eerie appearance once a year during the Kilorie festivity.

This legendary headless rider, known for his sinister presence, is in an insatiable quest to collect souls, which adds a layer of urgency to the twins' mission. As the Sullivan Detective Agency collaborates with the O'Brien’s and the experienced Mr. Amos Murphy, they quickly find themselves embroiled in a case that poses significant danger to their safety and unity. Along the way, they encounter a series of unsettling events, including missing friends, mysterious footprints that seem to appear out of nowhere, and shadowy enemies who harbor vengeful intentions.

What starts as a light-hearted summer excursion soon transforms into a whirlwind of peril and intrigue that spins the junior detectives into a tailspin of threats that test their wits and courage. Each clue they uncover brings them closer to the heart of the mystery but also deeper into the treacherous web woven by the Dullahan and those who wish to keep his secrets hidden. The summer sun may shine bright, but the shadows of this tale cast a long and haunting reflection, ensuring that this adventure is anything but ordinary.

Will these junior detectives manage to solve this case before the infamous criminal, Joseph Dunn gets his revenge or the monstrous Dullahan succeeds in its dark quest?

Double Trouble is an adventure filled with courage, friendship and pitfalls in the vast hills of Ireland.