There was one gap in the splendid entertainment laid on for King James I and his court on New Year's Day 1618: a masque called The Amazon by Lord Edward Herbert of Cherbury was advertised, but was called off at the last minute and never performed.

The cause of the cancellation has been buried in history but a true explanation may have emerged in a heavily crossed out and rewritten manuscript found in a trunk of old documents in a castle in Wales.

The manuscript shows The Amazon was not only never performed, it was never finished, possibly because Herbert got his plot into such a tangle.

The author, shown as boy band handsome in a famous miniature by Isaac Oliver, was the older brother of George Herbert, one of the most renowned metaphysical poets. He was also a lifelong friend of the playwright Ben Jonson and the poet John Donne.

He wrote on history and theology, but his literary fame rests on a swaggeringly boastful autobiography, published by the 18th-century connoisseur Horace Walpole. The book is a catalogue of beautiful women who lusted after him and men he clobbered in duels. At the coronation of James I, he wrote, "I could tell how much my person was commended by the lords and ladies that came to see the solemnity".

Herbert was clearly struggling with The Amazon, however. Felix Pryor, the manuscripts expert for Bonhams auctioneers who identified it, says: "The play was, in several places, so heavily redrafted and worked over that it was sometimes heavy going."

Pryor was looking through 17th-century property deeds when he found a bundle of papers in Herbert's handwriting, including The Amazon.

It opens with two Amazon girls discussing the general uselessness of men – as if, according to Pryor, they were "fresh from some 21st-century university campus". The women highly favour divorce: "Yf they did require divorse/ They might enjoy it, wthout mor remorse/ of doinge ill, than gamesters that give ore/ When they are losers".

Pryor said: "They'd meet up once or twice a year with the neighbouring males for breeding purposes, otherwise the Amazons spent the rest of the time bashing the hell out of them. Which, as one of the women remarks, shows just how stupid men are."

The plot becomes tortuous, involving pirates and the kidnapping of the king's sons, but ends surprisingly in a song in praise of love sung by two young men.

"All this seemed to place it not so much in the world of the court masque as in that of Shakespeare's late romances with their missing children and miraculous reconciliations," said Pryor, who predicts it will "keep academics busy for a long time".

The manuscript will be sold at Bonhams on 10 November. Estimates suggest it will fetch up to £90,000.