The eponymous dial first showed up in a series of 1960's comic stories fondly remembered by Mieville in his essay in the collected edition. Dial 'H-E-R-O' on the dial and one becomes a superhero for a brief time. A completely random, often weird superhero. The dials don't repeat heroes, so far as we were ever shown in the original series.
Mieville takes this initial concept and builds up an architecture of myth, legend, conspiracy, and science-fantasy weirdness around it. Overweight, 30ish Nelson Jent accidentally dials one of the dials. He becomes a superhero just in time to save his own life from some thugs who are after a criminal friend of his. This first hero is Boy Chimney, strange wielder of smoke and soot. There will be many others, from Captain Lachyrmose to Open-Window Man. There will even be a Native-American stereotype of a hero so ridiculous that Jent will hide for the duration of the change.
Others are searching for the dials. One of these searchers is a dial auto-didact who has her own dial. She's really the co-protagonist of Dial H. She's also a woman in her 60's who calls herself Manteau. So the protagonists of the comic are an overweight guy and a woman in late middle age. And the ultimate villain of the second half of the volume is a Canadian superhero turned super-villain. Several issues take place in Ottawa, Canada. Mieville has pretty much up-ended all the norms of a superhero comic book.
There's a width and breadth of invention here that will be familiar to those who've read Mieville's fiction. Things are a bit lighter and more hopeful here than in, say, Mieville's New Crobuzon or his London of King Rat-- the weird heroes of Dial H really are heroic, despite their frequent misgivings. There are apocalyptic stakes and strange monsters. There's world-hopping and dimension-hopping. There's even an issue that tips a hat to Simon and the Land of Chalk Drawings.
The art by Mateus Santolouco, David Lapham, and Alberto Ponticelli doesn't always serve the story. Mateus Santolouco, who illustrated the first few issues, is a fine and detailed renderer of weirdness, but his panel-to-panel progressions and in-panel storytelling can sometimes get confusing. Lapham cleaned things up when he took over for a spot.
Alberto Ponticelli, working with inker Dan Green, took a couple of issues to hit his stride. When he did, though, the book managed the combination of weirdness and easily followed graphic storytelling that it needed, peaking with that Simon and the Land of Chalk Drawings homage, an issue in which Open-Window Man spends most of his time talking to a sentient chalk drawing on a wall.
As satiric, ironic, and critical as Dial H can be of certain superhero maxims and stereotypes, it nonetheless concerns itself with the basics of superheroing more completely than an awful lot of non-weird superhero books and movies. Nelson Jent and 'Manteau' diligently protect innocent bystanders at every turn, no matter how awful the enemy they face. They don't destroy cities to apprehend one person.
And they're primarily motivated by curiosity about the dials and a desire to do good. Jent initially has a personal motivation, but that's resolved fairly early in the series. After that, it's all about the joys and responsibilities of superheroing, even when the superhero you're going to be for the next few hours is a sentient colony of plankton or a giant rooster with wheels for legs. People need you. Dial H. Highly recommended.