Sirens is famous for the revelation of its plot - everything in human history has been for just one purpose, so a messenger from the planet Tralfamadore can carry his message across the galaxy. Earth is just an unfortunate pit-stop when the ship breaks down on the way and everything on Earth simply is no more than the cosmic A.A. man's mobile phone - 'The meaning of Stonehenge in Tralfamadorian, when viewed from above is "Replacement part being rushed with all possible speed."'
However, not everyone knows that they are just a binary digit in this galactic switchgear, and the one man who is close to knowing - Winston Niles Rumfoord - has passed into the chrono-synclastic infundibulum and only materialises in Newport, Rhode Island, once every fifty-nine days. However, presumably using a better than average personal organiser, Rumfoord can plan and perform the long torture of Malachi Constant (once the richest man on Earth) and eventually lead him off to become part of an army on Mars; knowing that the Martian army will be wiped out when it attempts to conquer the Earth.
Constant will eventually escape to Titan, be rescued and be returned to Earth by Salo the Tralfamadorian, and then experience another surprise. I leave that one to you.
Oddly, enough, ignore the spaceflight and the plot is almost identical to Orwell's Nineteen Eighty-Four, with Rumfoord standing in for Big Brother. In both books the main characters have no chance - Winston Smith and Malachi Constant have everything stacked against them. Things are the same in Airstrip One and the Solar System. Or as Vonnegut wrote later, 'So it goes.'