Submarine spies and the unspeakable thing

When Caitlin discovers a dog on the banks of Wales' most sneaky river, she enters a world of spying, double-talk and mystery. Can Caitlin, who isn't very old, save the world, help the dog and rescue parrots, pigeons and people from an indescribably dangerous threat? As Christmas approaches and the decorations go up in her mum and dad's café, will Caitlin be able to keep her own secrets?