This is one of those Asimov books that gets recommended alongside Foundation and the robot novels but isn’t really doing the same thing at all, and I think that’s why I liked it more than I expected to. Three parts, barely connected on the surface. Part one is scientists on Earth discovering they can pull free energy across a barrier between universes, and slowly realizing the exchange is destabilizing the sun, and everyone with a financial or reputational stake in the tech doesn’t want to hear it, which, fine, that’s a very of-its-moment environmental parable but it’s a solid one, ends on a sharp little gut punch.

Then part two switches entirely to the alien side of the exchange, told from the para-universe, and it is genuinely one of the strangest, most fully imagined alien societies I’ve read, three-sexed beings called Rationals, Emotionals, and Parentals who periodically merge into a triad in a process the book describes with more tenderness than you’d expect from 1972 Asimov. It’s the part people talk about for a reason, it does not read like anything else in the genre, and it’s carrying almost the whole book on its back. Part three, back on the Moon, has some fun ideas but it’s the weakest leg of the three, romance plot that a few people I’ve talked to found a little flat, resolution that wraps up faster than the setup earned.

Where it lost a full star for me is that ending, the alien section just kind of gets left behind once the plot resolves back on our side, no real payoff to the culture we spent forty pages falling in love with. Still, a book that’s mostly one incredible novella wearing a decent frame around it is still worth your time. Four stars, read it for the middle third alone if nothing else.