This is a bit of an old, silly one, so the general premise is that humans discover a way to make portals to other worlds - the only catch is that the forces of physics limit them to just a few inches in diameter.

The difficulty is in trying to fit a cosmonaut through something the size of a catflap.

On the other side of the universe, a small drone whirred through the alien atmosphere, trailing a cable behind it that led to the portal to Earth.

“Nice flying. Manmade horrors beyond our comprehension, just beyond the next hill,” the scientist said to the drone pilot, seated in a comfortable office at Cape Canaveral. “Try to stay just above the vapour clouds. They knocked two of ours out of the air just yesterday. Little critters seem to be collecting them.”

“I’m amazed they can see anything through this,” said the drone pilot, squinting at the live footage. The alien atmosphere obscured everything with drifting purple vapour.

“The eyeballs certainly can. And if one sees you, so do the rest.”

“They can communicate?”

“In a sense. We’re still not quite sure how.”

The severants were a miracle of bioengineering, the genius of their design matched only by the complete failure of their implementation. Only their creator, Professor Bryant, knew the full details of their creation and functioning. It was cosmic irony that the only mind that understood them should be trapped within them.

The drone was scanning the radio waves. Behind the constant, insect-like chittering of the hive, as they exchanged bio-electric data, was the garbled, distorted human voice of Professor Bryant.

“…e-sc… esc… KRRCH …ape… es-…ha-have t… t-to… c-can’t… c-can’t…e-esc… SSSHH …ape…”

They’d been observing the planet for months now. The Professor’s words had grown more frantic recently. The poor man. When he’d first divided himself into 2,301 pieces, he’d been able to control each part separately. Now there were millions; he had no hope at all. The endless sensory input from a million disobedient bodies must have been beyond comprehension.

The severant settlement came into view. The researcher could have sworn it had grown since the last time he’d seen them. You could almost see the insect DNA Bryant had woven into their genetics, as the severants scurried about it like a nest of ants, endlessly busy: the fingers crafting, the eyeballs standing on guard duty, the mouths masticating nutrients into a slurry for the others to slurp up.

Individually, the severants were not intelligent, about as smart as a very stupid gerbil. Each contained just a small clump of nervous tissue, programmed only to eat and breed, and eventually reassemble themselves into a fully formed human (though that final instinct had evidently gone wrong somewhere). But as a collective, they could organise themselves. For especially complex matters, they even seemed able to access the mind and memories of Bryant, treating him like an external hard drive of knowledge. A man like Bryant knew almost anything.

As the drone flew over the village, the radio waves screeched. “…escape… I… I c-can’t… I can’t… h-hold th-”

The 'breeder' of the hive, a penis on legs, turned its eyeless body as if looking up at the drone. It wobbled its turgid body excitedly.

“Pull up!” the researcher gasped. But it was too late. A hundred tiny spears clattered into the underside of the drone, clogging the rotors, and it crashed to the ground, lying on its side with a cracked camera.

“Ah, darn it,” the researcher shrugged, patting the pilot on the shoulder. “No worries. Those drones only cost a few thousand dollars. We’ll get another prepped.”

The drone pilot was still staring at the screen. The finger-severants were clambering over the drone, working collectively with their finger-shaped bodies to begin tearing the machine apart. Just behind them could be seen a lashed-together pile of scrap metal. One severant pried one of the batteries from the drone, scurried to the pile, and slotted the battery into it. Above the pile, a Planck portal winked into existence.

“…th-they a-are… tr-trying… esc-ape… I… c-can’t… st-stop th-them…” screeched Bryant, as the tribe of severants began to leap through the portal.