The saga of the Deptford Mice continues in this sequel to The Dark Portal. The evil rat Jupiter has been defeated, and the young Deptford mice emerge from the sewers to start a new life in the country. But the albino mouse Oswald is seized by an illness and lies near death. His friend Audrey enlists the help of the ancient squirrel mystic Starwife, who agrees to heal him on one condition. She reintroduces Audrey to the once villainous rat Madam Akkikuyu, whose mind is now broken, and decrees that Madam Akkikuyu must stay with Audrey until she dies.

In the country, the mice find a bucolic existence, until they find themselves embroiled in a series of murders. The country mice suspect that headstrong city mouse, Audrey, is the culprit. But the truth is far more sinister, as Audrey witnesses the return of the supernatural evil she thought had been destroyed.

Every mouse knows: Beware the Grill.

On the other side of the Grill lie the sewers, where bloodthirsty gangs of rats roam the tunnels. It is in the depths of this blackness that the mysterious god Jupiter has his lair.

But one curious mouse can’t resist the perilous world beyond the Grill, and soon no mouse can ignore its dangers any longer. Before the alter of Jupiter himself, a small band of mice must face evil at its strongest, or the entire city of London could be destroyed.

The hideous spirit of Jupiter, the evil cat who was once lord of the sewer rats, has returned, more terrifying than ever. Bent on revenge for his death, he is determined to turn the world into an eternal winter of hatred. The Deptford Mice feel his chilling grasp everywhere they turn. Their allies are murdered, and the prophetic bats flee, sensing impending doom. A rat army thirsty for mouse blood is building underground. And most frightening of all is that the Starglass, their magical source of knowledge and power, has been stolen by a cruel and violent hand. The mice know they face the most desperate struggle of their lives. Will anyone survive?