In the sewers of Deptford there lurks a dark presence that fills the tunnels with fear: Jupiter, a dark god known only by his evil, no one really knows who he is except for his lieutenant Morgan his glowing eyes seen behind a dark "portal" in the tunnel wall. The rats worship him and do his dark bidding.
The Deptford mice, who worship the Green Mouse who brings Spring, are aware of Jupiter's evil presence in the tunnels. One of the mice, Albert Brown, is pulled through a grate into the sewers by the dark enchantments that Jupiter has woven there. He meets a cheeky mouse from the city, Piccadilly, also lost inside the lair of the rats. When Albert is captured by Morgan, a Cornish, piebald rat with a stump for a tail and rings through his ears (none other than Jupiter's lieutenant) and brought to Jupiter, Piccadilly escapes into Albert's home, known as the Skirtings, where he meets a large number of friends.
Firstly, there is Audrey Brown, Albert's day-dreaming daughter who hates Piccadilly for surviving where her father did not. Then there is Arthur Brown, Audrey's plump and funny brother. There is also simple fieldmouse William Scuttle or 'Twit' and his cousin, the sickly albino runt, Oswald Chitter. Along with the old Midship-mouse, Thomas Triton and Gwen Brown, the mother of Audrey and Arthur, they venture into the sewers to banish Jupiter from the world. The Rat God, however, has an evil plan to unleash the Black Death on the world once more, ridding it of humans and enabling him to conquer it.
After a tense-confrontation with Jupiter in this throne-room during which the malevolent deity is revealed to be a monstrous cat, Audrey defeats the Dark Lord by flinging her mousebrass at him. The talisman explodes and Jupiter falls into the water that is rapidly flooding his chamber. The spirits of the many mice and rats he has tortured and devoured then rise up and drag the dark god down to a watery grave. Audrey glimpses her father among the legion of spirits as they depart and despairs, realising that he is truly dead. The sewer floods and the Black Death is destroyed. Thankful that they are alive and resolving to celebrate Jupiter's downfall once they return home, the mice begin the journey back to the surface but as they leave, Orfeo the bat foretells doom.
- ↑ Text on the front cover of the 2000 Hodder Silver edition of The Dark Portal.