Gremlins were a race of hungry beings that existed one millisecond out of sync with the worlds they invaded. This caused them to be invisible to practically everyone.
In 1894, over a thousand Gremlins came to the Royal Observatory in Greenwich looking for something to gnaw on. Strax heard their high-pitched chatter on his internal comm unit for several days. About a week after the Gremlins came to Greenwich, William Christie contacted the Paternoster Gang and asked them to help him find why there were bite marks on the telescope's lens. Vastra had a magnifying glass that could see through time and she was able see and talk to the Gremlins. Jenny Flint negotiated with the Gremlins and had them clean up all the damage they did to the observatory. The Gang then helped the Gremlins settle at the bottom of the River Thames, where they were able to gnaw on all the junk they wanted. (PROSE: The Curious Case of the Miniature Menaces)