Reviewed date: 2019 Jul 24
Roggan: a long time ago
Nearly two thousand years ago, the two nations of the watery planet Roggan jockey for resources. Their world lacks sufficient dry land--only one small continent rises above a vast globe-spanning ocean. Then, armageddon: nuclear weapons destroy the enormous polar icecaps and release the vast amounts of water, flooding the entire surface. All of Roggan is now sea. A few survivors start a new, underwater civilization.
Hydro: present day
Dennis Dillard is a professional feeler. That is, he records his emotions onto a tape, and the recordings are played back for entertainment by the masses on Earth. Dillard's job is to seek out new and strange experiences, and thus, he's made his way to the strange watery planet Hydro. (Hydro is Roggan, which we know, but Dillard and the rest of the humanity do not. Presumably the Roggans are still there, somewhere, under the sea.)
The spaceport facilities on Hydro--actually, all the facilities--are built on floating platforms anchored to the seabed by cables. It's a tough world, but water is a valuable resource, so when Hydro was first discovered by the Venusians fifty years prior, they quickly established the floating base. Now the planet is shared between Earth and Venus.
Venus vs. Earth
The Venusians are not aliens, they are humans exiled from Earth several generations ago. A group of intellectual elites tried to stage a coup, to take over control of Earth and remake it into a new, better society. Rather than imprison or execute the plotters, the ruling authorities on Earth exiled them to Venus. Conditions were hard on Venus, and the Venusian people are a hard race. Generations later, there still exists bitter hatred among Venusians. They want the right to return and live on Earth, which Earth will never permit--Earth itself is dangerously overcrowded as it is.
The Professor, the Mysterious Mara, the Venusian spaceship
On Hydro, everything goes wrong for Dillard. First he pays a visit to Dr. Stanley, an old professor who used to belittle Dillard back at university, but the professor is missing. He encounters only the professor's mysterious new assistant Mara Hunt, a disagreeable and harsh woman who questions Dillard pointedly. Next, Dillard tours a Venusian spaceship--a unique experience that makes a great recording--and meets an attractive and captivating Venusian woman. But the tour is abruptly ended and the spaceship explodes moments he disembarks. Dillard goes to drown his sorrows by availing himself of the local attractions Hydro offers (which are substanstial, due to a quirky legal loophole) but his dinner is interrupted when he's rudely kidnapped by aliens.
Kidnapped by Roggans
The kidnappers are Roggans. They take him to their undersea base, where they question him. Dillard learns that that Roggans and Venusians have secretly formed an alliance and intend to attack Earth. Both Venus and Roggan desperately wish to take Earth's land for themselves. Mara has also been kidnapped, and as for Dr. Stanley, he'd been working with the Venusians and Roggans.
The truth: Earth's military dominance
After much perfunctory excitement (various escapes and recaptures) Dillard gets an audience with the authorities of Roggan and Venus. Dillard explains that truth: Earth's economic and military power is so overwhelming that any Roggan or Venusian attempt is doomed to failure. The only outcome of a war will be that Venus and Roggan/Hydro will be quickly rendered uninhabitable.
A peaceful way forward
Dr. Stanley offers a peaceful solution. He has invented a device that transmits water across space via radio waves. With this technology, Roggan's oceans can be drained and the Roggans can live on dry land on their own planet again. And Venus can use the device to reduce the heavy water vapor in their atmosphere, making their world more hospitable. It's a win for everyone! Everyone agrees to this solution.
Dillard wonders whether this is a long-term solution. The Roggans have destroyed their world once already, and how long before they fill up their planet and come looking again at Earth? No worries, says the Dr. Stanley. He's studied Hydro, and the planet is unstable. Its mass is only just barely able to keep its core contained. As the Roggans drain away their oceans, the mass of the planet will be reduced. If they drain away too much in an attempt to get more dry land--and they surely will--their planet will rip itself apart. The Roggan menace will solve itself.