Five Are Together Again by Enid Blyton

A close-knit group of cousins and their dog reunite for a holiday that quickly turns into a mystery-filled adventure; while exploring the countryside and their familiar haunt, they stumble upon suspicious goings-on and a hidden danger. Relying on teamwork, cleverness and courage they follow clues, outwit the villains and bring the truth to light, with the loyal dog playing a key role in saving the day.