You’ve gone with the family for a picnic into the countryside. You park the car, walk a mile or so to a scenic spot, lay out the picnic…and then the heavens open. At first you try to tough it out but there’s no sign of the rain abating and, sooner rather than later, you give up and trudge back to the car, damp and hungry. But, of course, in the rain and confusion, you didn’t clear up as you might have. A sandwich box fell out of the bag. A glass bottle was left open on the floor. Some paper, some plastic wrappers, some of the food that you’d started eating and maybe even an old MP3 player, forgotten in all the confusion.

You pile back into the car and resolve, on the way back, to try a country pub next time. But as you drive away, the creatures that live around that scenic spot look out from their hiding places. They look around, checking that you’ve gone. Then, some of the bolder ones, creep forward and start investigating what you left behind. A sparrow makes off with the crumbs from the sandwich. A bee discovers the spilt Coke and gorges on sugar until it collapses in a sugar faint. An ant crawls into the open glass bottle and gets stuck there, the sides too smooth for it to climb back out, while its comrades wave their feelers at it from outside, entirely unable to rescue it.

That’s the premise of the Strugatsky brother’s novel – only we’re the ants, the bee and the sparrow. Aliens, so far beyond us that they don’t even notice we’re here, land on earth, spend a short time here, and then leave – leaving stuff behind. Some of it is dangerous, some useful, some incomprehensible.

Science fiction is a literature of ideas and this, at the time wholly original idea, is perhaps the best attempt to describe what a real alien visit might be like. As such, the novel is almost extraneous to the idea, serving largely to showcase the idea. The idea itself has proved very powerful, entering the SF vocabulary. The novel from which it came is suitably indeterminate because the visit was indeterminate: true to the idea perhaps slightly less satisfactory as literature. But, in SF, the story serves the idea, not the other way round.