It is set in the Middle Kingdoms, particularly Darthen and Arlen. It concerns the protagonists' fight to keep the Shadow from taking over the Middle Kingdoms. The main characters are Herewiss, the first man in a very long time to wield the power of blue Fire; his lover, Freelorn, the rightful heir to the throne of Arlen; Herewiss's other lover, Sunspark, a fire elemental; and Freelorn's other lover Segnbora, sorcerer/bard/swordswoman.
- The Door into Fire (1979)
- The Door into Shadow (1984)
- The Door into Sunset (1992)
In 2018, Diane Duane revived the series with Tales of the Five, a series of five books (ranging in length from novelette to novel) which will lead up to the long-awaited conclusion, The Door into Starlight. Books in this series:
- The Levin-Gad (2018)
- The Landlady (2019)
- The Librarian (estimated for Q3 2022)
All the books are available as eBooks from http://www.dianeduane.com/.
Other works by the author set in the Middle Kingdoms are the Sirronde's World series and the novelette Lior and the Sea.
This series contains examples of:
- Absurdly Sharp Blade: Skadhwe.
- Absurdly Youthful Parents: The Goddess's teachings dictate that having children is important, but separate from marriage (in order to allow people both to be fruitful and marry for love)—in fact, people aren't eligible to marry until they've had children. So most people get the babies thing out of the way as soon as possible, leading Herewiss to have nine-year-old sons in his mid-twenties.
- Angel Unaware: The innkeeper they meet, though they do figure out who She is eventually (after comparing notes and realizing they all slept with her at exactly the same time).
- Anti-Villain: Cillmod turns out to be this.
- Be Careful What You Wish For: Segnbora wanted to be different...
- Blazing Inferno Hellfire Sauce: Whitefruit, which is similar to the capsicums we know and love, is particularly prized in Steldin, with certain cultivars taking it Up to Eleven.
- Blessed with Suck: Blue Fire could be considered this, because using it uses up your life.
- The Dragon: Rian.
- Everyone Is Bi
- Fighting a Shadow: Fighting the Shadow, in fact, and it can't be destroyed. "Shadow" isn't just a Dark Is Evil metaphor; it is literally the shadow of the Lover, and will exist as long as he does.
- Gender-Restricted Ability: The blue Fire; Herewiss's possession of it is universally met with shock (and sometimes horror, especially from more rigid-minded Rodmistresses), and he has a lot of trouble getting properly trained because of this.
- God's Hands Are Tied: The Goddess needs the characters to act for her.
- Good Is Not Nice: The Goddess sometimes.
- Half-Human Hybrid: Segnbora sort of becomes this.
- Happy Place: Segnbora can go into the part of her mind where the Dragons are.
- The Hecate Sisters: The Goddess, who is a true-to-the-archetype Maiden/Mother/Crone three-in-one.
- I Know Your True Name: People have powerful, secret true names. Plenty of people don't know their true name, though.
- Interspecies Romance: Herewiss and Sunspark; Segnbora and Hasai.
- King Incognito: Freelorn for much of the series, Eftgan at one point, and Herewiss, who is a prince.
- Mandatory Motherhood: Everyone must have a child before they can get married.
- Meaningful Name: Segnbora becomes Freelorn's standard-bearer.
- A Mind Is a Terrible Thing to Read: Segnbora complains about hearing everyone's thoughts.
- Nice Job Breaking It, Hero: Nice job letting out the soul-eating monsters, Herewiss.
- Our Dragons Are Different: They're immortal by reincarnating (sort of - it's complicated) into their children and from another planet.
- Pieces of God: Combined with I Know Your True Name — the Goddess's true name is made up of the true names of every person and thing in creation.
- Polyamory: It's accepted and normal. Almost all the main characters get married to each other in the end of The Door into Sunset.
- Power Glows: Both of the magic swords glow with blue fire.
- Rape as Backstory: Segnbora
- Red Oni, Blue Oni: Freelorn and Herewiss, respectively.
- Rightful King Returns: Freelorn in the end of The Door into Sunset.
- Royal Blood: Only those of royal blood can do everything necessary to be a king or queen.
- Royals Who Actually Do Something: Freelorn, Eftgan, and especially Herewiss.
- Satan: The Shadow is basically the Devil.
- Telepathy: Called "bespeaking" if they're both talking on purpose and "underhearing" if not.