In an infinity of parallel worlds, many are the inventions which would paralyze less advanced worlds. And one such invention was the one the noted physician Jome Kenard used to restore life and senses to the injured beauty Allyn Vage. His strange machine had been imported from a seemingly primitive people on the world of Akkilmar. It worked - but its workings were incomprehensible even to Earth's best scientist. Nevertheless, once used, the doctor and his patient had been transformed into the lever that could topple a world!

This is one of Brunner's better books - that explores the parallel worlds idea and takes it in unusual directions. (The 1961 Ace Double was original - the first publication in any form.)