Her unnamed heroine, kidnapped by sea from her home, is blown by storms to the North Pole and thence to another world which adjoins ours there. The inhabitants immediately make her their Empress, and we then settle down for a hundred pages or so of exposition and world-building, some of it a little satirical, some simply speculative and imaginative (some of it perhaps inspired by her visit to the Royal Society the previous year). The Empress then causes further point-of-view confusion by inviting the Duchess of Newcastle to come visit her on her own planet, and, using otherworldly technology, exterminates all of England's military enemies to ensure that Britain can be Top Nation.
It's a undisciplined, rollicking, diverting ramble through the mind of one of the era's most interesting personalities, and I'm really surprised that it is not better known - I think I came across it only browsing Wikipedia, though I then found an essay about it in Speculative Fiction 2012 when I was already half way through. I also detect one or two elements which surely Swift must have put directly into Gullver's Travels; he would surely have known and read this.