A leaked 22-year-old letter from Game of Thrones author George RR Martin reveals the author’s earliest plot ideas for his epic series of fantasy novels, which included a disturbing love triangle that failed to make the final cut.

Written to his agent, the late Ralph Vicinanza, the letter is dated October 1993, and in it Martin pitches “the high fantasy novel I promised you”, A Game of Thrones.

Unaware of the major writing task which would lie ahead of him, Martin told Vicinanza: “When completed, this will be the first volume in what I see as an epic trilogy with the overall title, A Song of Ice and Fire.”

The book series now stretches to five increasingly lengthy volumes, with two more yet to come. The letter goes on to sketch out Martin’s plans for how the series would play out. Affixed to the walls of his UK publisher HarperCollins’ new London offices – with a potentially revelatory final paragraph blacked out – it was photographed and leaked online by Waterstones yesterday.

The tweet was later deleted by the bookseller, but HarperCollins confirmed that the letter was authentic on Friday. “We’re enormously proud to have it in our possession,” said the publisher.

The images were captured by the fan site Winter is Coming, which blogged that “it’s super-interesting to see the specific ways that the story-as-plotted changed, and to try to figure out why.”

Martin explains to his agent how “the enmity between the great houses of Lannister and Stark [will play] out in a cycle of plot, counterplot, ambition, murder, and revenge, with the iron throne of the Seven Kingdoms as the ultimate prize”.

Overseas, meanwhile, he writes that a “second and greater threat” is taking shape, “where the Dothraki horselords mass their barbarian hordes for a great invasion of the Seven Kingdoms, led by the fierce and beautiful Daenerys Stormborn”. And in the north, “the greatest danger of all” awaits, “where half-forgotten demons out of legend, the inhuman others, raise cold legions of the undead and the neverborn”.

So far, so similar to how his series is currently playing out, and Martin also names his “five key players” as Tyrion Lannister, Daenerys Targaryen, and three of the children of Winterfell, – Arya, Bran, and the bastard Jon Snow.

But he reveals an unexpected original plan for Arya, who is nine when the series opens. Martin writes: “she realises, with terror, that she has fallen in love with Jon, who is not only her half-brother but a man of the Night’s Watch, sworn to celibacy. Their passion will continue to torment Jon and Arya throughout the trilogy.” The character Tyrion, according to the letter, also falls “helplessly in love with Arya Stark while he’s at it. His passion is, alas, unreciprocated, but no less intense for that, and it will lead to a deadly rivalry between Tyrion and Jon Snow.”

Other plot points have also fallen by the wayside in the two decades since Martin wrote the letter – but at least one of his original assertions is still in force. Infamous today for his willingness to kill off major characters without a backward glance, Martin wrote in 1993: “All three books will feature a complex mosaic of intercutting points of view among various of my large and diverse cast of players. The cast will not always remain the same. Old characters will die, and new ones will be introduced. Some of the fatalities will include sympathetic … characters. I want the reader to feel that no one is ever completely safe, not even the characters who seem to be the heroes. The suspense always ratchets up a notch when you know that any character can die at any time.”

The novelist went on to publish the first novel in the series, A Game of Thrones, in 1996. He followed it with A Clash of Kings in 1998, A Storm of Swords in 2000, A Feast for Crows in 2005 and A Dance with Dragons in 2011. Last week, HarperCollins confirmed that the sixth book in the series, The Winds of Winter, is not in its publishing schedule for 2015.