It opens in the world of Big Science that Asimov knew well, full of in-fighting and the race to publish first. The Inter-Universe Electron Pump sucks unlimited energy from nothing, making all power stations obsolete and bringing a new golden age. No one--especially not the scientist who got the credit--wants to listen to the doomsayer Lamont who calculates that the pump's side effects may detonate the Sun. Worse, there's no kudos for him: "And no one on Earth will live to know I was right".
Part two moves to the dying parallel universe whose hyper-intelligent aliens actually invented the pump and don't care what happens to our Sun. Asimov cleverly focuses on three immature aliens whose intelligence is less daunting and who slowly learn--with very different personal reactions--about their race's weird analogue of sex, about the pump's moral implications, and eventually about the unexpected meaning of maturity. These are the most original, engaging aliens Asimov ever created.
Part three is set in a carefully worked-out Moon colony and grapples with the "para-physics" of inter-universe loopholes. Can a politically acceptable replacement for the pump be developed? Solid, workmanlike SF with far more talk than action: one of Asimov's rare standalone novels. --David Langford