The series takes place in Victorian England, in a time before the peace treaties between the Shadowhunters and Downworlders existed. As Shadowhunters consider themselves superior or purer than Downworlders or demons, they may have no qualms about killing either.
The series follows Tessa Gray, an orphaned teenage girl who discovers she has the power to shapeshift, but doesn't bear a mark that shows she is a warlock, abilities that plunge her into a world she never knew existed, and she will have to learn to master them if she wants to find her brother. To do this, she will have to forge an alliance with some Shadowhunters if she wants to survive in this dangerous world.
- Theresa "Tessa" Gray — sixteen-year-old protagonist of the series who has lived normally for most of her life until her brother is apparently held captive in London by the women who introduce her own abilities to her, inevitably pulling her into the world of Shadowhunters and demons. Despite often being referred to as a Downworlder, she is infact a Shadowhunter-Demon hybrid.
- William "Will" Owen Herondale — a Shadowhunter from the London Institute who finds himself drawn to the Downworlder Tessa.
- James "Jem" Carstairs — Chinese-British Shadowhunter whose parents were killed by a demon called Yanluo who lives on a drug that slowly kills him.
- Jessamine "Jessie" Lovelace — Jessamine is one of those Shadowhunters who detest everything about being one. Despite this, Jessamine can be dangerous if she wants to be—to herself as much as to the supernatural creatures she's sometimes forced to encounter.
- Henry Branwell: absent-minded and brilliant, Henry spends his waking hours in the crypt of the Institute, inventing fabulous machines and weapons out of cogs, cams and gears. Unfortunately just as many of Henry's inventions don't work as the ones that do, and Henry is as likely to set himself on fire as he is to invent something new and amazing.
- Charlotte Branwell: twenty-three-year-old Charlotte runs the entire Institute with a capable hand. Kind and loving, she does her best to care for the orphaned Shadowhunters who live under her roof whilst hiding her loneliness from the absent-minded husband she loves. It seems as if there's nothing Charlotte can't manage — until Tessa's search for her missing brother begins to uncover a web of corruption and deceit in London's Downworld that threatens to tear London's Enclave of Shadowhunters apart.
The Clockwork Angel audiobook was read by Jennifer Ehle, Clockwork Prince by Ed Westwick and Heather Lind, and Clockwork Princess by Daniel Sharman.
- The series is very loosely modeled after A Tale of Two Cities.