Story in The Hastur Cycle by James Blish. The narrator meets with Bill Atheling, who has recently married the artist Samantha Brock. They discuss Robert W. Chambers and his book, The King In Yellow. Bill has gained a copy of the Play, allegedly written by Chambers, via H.P. Lovecraft (although the narrator suggests that Bill may have written it himself). The Dream-Leech is intended as a sequel to this story.
In More Light, the play is set in Hastur, beside Lake Hali. Ruled by Queen Cassilda, the city is at war with the rival city of Alar. Both cities are engaging in a lengthy siege with the other, causing much tension.
Surrounded by her children (Prince Uoht, Princess Camilla, and Prince Thale) and her grandson (The Child, whose parentage is never stated), Queen Cassilda desperately searches the city for 'the yellow sign', a symbol that will herald the return of the 'The King in Yellow', a mythic figure whose return might turn the tide of the war in Hastur's favor. Her chief priest, Noatalba, has found a masked man named 'The Phantom of Truth', who wears the yellow sign on his robe and who alerts her of the truth about the 'King in Yellow'. Hailing from Alar, the Phantom of Truth alerts Cassilda that the the King in Yellow is coming not to save the city of Hastur but destroy it. The only way to stop him is to wear a special mask, the 'Pallid Mask', like the one on the Phantom's face. The mask represents 'truth', a weapon that can be used to fight the the King in Yellow's seductive influence. Finally, he tells Cassilda that she should throw a party, with everyone wearing the pallid mask to draw out the The King In Yellow and reject him.
At the masquerade, the people of the city wear the pallid mask but ultimately remove their masks at midnight when the King in Yellow does not show himself. At this point, the Phantom of Truth reveals to Cassilda and Camilla that his mask is not removable; it is part of his face, as the Phantom had fused the mask over his face to protect himself. But it does little good, as the the King in Yellow reveals himself as one of the party goers and mortally wounds the Phantom. Having revealed himself, the the King in Yellow states his terms for sparing Hastur. They will declare war on Alar in order to prove themselves worthy of survival and ultimately conquer the planet for the King in Yellow. The King in Yellow in turn, will not return to claim his throne over the planet until Hastur has conquered the planet or been wiped out by its enemies. Noatalba thanks the King in Yellow for his mercy, which angers the King in Yellow. This causes him to exact one last concession from the people of Carcosa, for their alliance with the Phantom of Truth. From this moment forward, all residents of Hastur must forever wear the masks they wore at the masquerade, as must all future generations of Hastur live their lives perpetually masked. Cassilda reacts in horror as the King in Yellow leaves, in triumph. The final scene is the Phantom of Truth rising from the ground, horribly disfigured by the King in Yellow's attack with The Child following him off-screen.
Innovation in this version of the play
The big 'reveal' about the play in More Light is the revelation that, with the exception of the characters of the Phantom of Truth and the King in Yellow, it was written and conceived for an all-black cast. The play itself breaks for a scene where the narrator discovers a note placed in the script where Chambers discusses production of the play. The note explicitly states that Cassilda, the royal family, and Noatalba are black and their actors must be played by black actors.
In summary: Blish's play and its setting
Hastur is a city beside Lake Hali. The city is ruled by Queen Cassilda (who may be inferred to be the widow of King Aldones, after whom there will be no king in Hastur until the King in Yellow). Her children are Prince Uoht, Princess Camilla and Prince Thale. The character called The Child is stated to be her grandchild although its parentage is not given. A priest Noatalba is present. Alar is named as the inventor of the Pallid Mask; since 'Alar' is apparently a place in the play, presumably it means the city invented it. Blish also incorporates the famous line from The Repairer of Reputations, "He is a King whom Emperors have served" into the Second Act.
In the play, the Stranger is described as being from Aldebaran and an enemy (rather than an avatar or messenger of) the King in Yellow. The Pallid Mask is also implied to be an enemy of the King. Truth is said to have been invented in Aldebaran as a weapon against the King.
The King in Yellow's rule
There are several errors in the play (some of which Lin Carter corrected in his partial rewrite) namely, the First Act is not divided into two scenes, Cassilda's Song is relocated from the First to the Second Act and the name of Naotalba is misspelled as Noatalba. As these errors would be unlikely to exist in a manuscript written by Chambers, it is possible they were deliberate on Blish's part and likely they serve to support the narrator's contention that Bill Atheling is the 'real' author of this version of the play.