The first book in The Deptford Mice trilogy and Jarvis's debut novel, it follows the story of Audrey Brown, a mouse girl who is looking for her missing father.

When Audrey goes to get her mousebrass, she is shocked to encounter the Green Mouse, a benevolent deity worshipped by the mice, who personally hands her a pendant known as the Anti-Cat Charm.

That night, Audrey slips into the sewers to visit a rat fortune teller named Madame Akkikuyu, hoping she can tell her where her father is.

Arthur and his friends the albino mouse Oswald and fieldmouse Twit enter the sewers to search for the missing Audrey, eventually finding her and Piccadilly after they all fight off a gang of rats.

Waiting at the grill for them to return and regretting her callous treatment of Piccadilly, Audrey is kidnapped by rats sent by Jupiter, who is concerned about a prophecy he heard that the girl will defeat him.

Audrey is taken to Jupiter's altar chamber, where she has a tense confrontation with the malevolent deity, who emerges from his dark lair for the first time in hundreds of years to reveal himself as a hideous giant cat.

A group of mice including Piccadilly, Arthur, Gwen, Twit, and grizzled seafarer Thomas Triton show up in the altar chamber to battle Jupiter as well.

The evil cat is dazzled by brilliant green flames that explode from the mousebrass, losing his balance and falling into the water rapidly flooding his chamber.

The author conveys a sense of place powerful enough to elevate the South London boroughs of Greenwich and Blackheath to requisite stops on any bookish child's literary tour of the British capital.

"[14] The book also received a positive review in The Sacramento Bee stating that "this fast-paced suspense tale is all heart in its gripping story of good vs. evil and life-threatening confrontations beneath the streets of London.

"[15] Lloyd Alexander called The Dark Portal "a grand-scale epic" that is "filled with high drama, suspense, and some genuine terror",[16] while Madeleine L'Engle said that "Robin Jarvis joins the ranks of Kenneth Grahame, Richard Adams, and Walter Wangerin in the creation of wonderfully anthropomorphic animals.

"[17] Peter Glassman, owner of the New York City children's bookstore Books of Wonder, obtained a copy of The Dark Portal while on a trip to London.

[18] The author of The Outsiders, S. E. Hinton, once told Glassman that The Deptford Mice novels became her son's favorites after finding them in Britain, but she could not understand why they were not yet available in the United States.

[19] In the mid-1990s, there were plans for Jim Henson Pictures to make a film adaptation of The Deptford Mice, which would be based on the story of The Dark Portal and feature animatronic puppets.