Simon ("Widgie") and Jen Widgeon are innocently exploring the abandoned Highgate rail tunnel near their home when they discover a hidden gateway halfway along it.

It quickly becomes apparent that Widgie and Jen have become caught up in an entire underground civilisation, which the North London System had kept secret for centuries from the world above.

Leonard includes appendices relating the aftermath of the story underground, in which the Railwaymen and Canal Folk discuss the joint threats of the children and the rats, and giving a short history of the North London System (and suggesting that similar societies exist beneath many major cities, at least in England).

[citation needed] Leonard's secondary world is unusual as a result of the fact that it overlaps with the primary one and is separated from it, it seems, through nothing stronger than blind luck.

The children finally make their escape through an Observation Shaft, similar to the one they entered through, which leads into the Post Office Railway, and is used to study the operation of electric trains.

It serves as the spine of the canal network and the setting for one of the most dramatic scenes in the book, as both the train (with Widgie on board) and the boat (with Jen) race against time to reach and pass a bridge over it as the river becomes swollen and flooded.