Neptune King and the Mysterious Monk is the 27th book in the series. They're hired by Friar Luigi, a Capuchin monk from a monastery in Devon. Ever since a collection of rare books, diaries and manuscripts arrived at the monastery there has been strange goings on in the night. Each morning they find the library in disarray; the books scattered about or misplaced and sometimes strange noises in the night. When they go to investigate they find nothing and no one. This is puzzling enough, causing some of the monks to believe there is a poltergeist in their midst, but when elderly Friar Matteo is found dead at the foot of the stairs, his skull stove in, Friar Luigi suspects foul play.

He travels to London to enlist Neptune and Shark's aid, which they are more than happy to offer free of charge. Neptune is ecstatic at this opportunity to disguise himself as a monk and go under cover in the monastery as Friar Benedict, while Shark backs him as a carpenter doing necessary repairs to the ancient monastery. He brings his dog Fang along with him, who turns out to be a great hit with the monks and useful too as they put him in the library overnight. They have a couple of suspects-two of the newly arrived monks and when one of them presents the next morning with a torn robe and Fang has a piece of a robe in his mouth, that monk goes to the top of the list, although Neptune has doubts as to his guilt. When an attempt is made on Luigi's life, they double their attempt to find the culprit in no doubt at all that Friar Matteo's death was murder. A new suspect emerges after they prove he lied and Shark telephones Neptune's wife, Rachel to get her to undertake a background check on him. That's held up when Rachel collapses and is found unconscious in her office by Milo, one of their employees. Neptune rushes back to London to be by her side, learning that she had a very close call and that both she and the baby were at risk of dying.

With treatment Rachel improves but is ordered to rest at home and to do nothing strenuous that could put her blood pressure up until the baby is born, which much to Rachel's dismay is not for another six months. Neptune doesn't want to return to the case, but Rachel insists upon it. He's a little distracted with worry and not at his best; despite that they still manage to solve a 160 year old mystery of missing Papal jewels and catch the killer but not without risk to life and limb when they're entombed in the labyrinth of underground tunnels that start from the ossuary at the monastery.