HOOK LINE SINKER: Giant rats invade a small to medium wilderness town.

[Shorty Lickens]

I forget what qualifies as HLS so I'll just lay it out my own way.
PC's are in a small or medium town deep in the wilds of North America, far from any major nations. Whilst negotiating over the price of essential goods they see a small gang of normal rats scurrying by. Shopkeeper tells them they are common but not dangerous and too numerous to be cleansed easily. A few minutes later rats the exact same color and shape but much larger run by. They look much scarier being larger but again the shopkeeper tells them they are not too rare and also haven't been very dangerous so far.
Later on the street or in a bar several VERY LARGE rats race around and attack a small child, possibly killing him/her if the PC's don't have a Juicer or Crazy or prescient psychic in the party. Or perhaps they have a very fast pet with excellent smell or hearing who finally gets a chance to shine. The locals have limited weaponry and don't want to fire lasers in town for fear of destroying entire buildings or vaporizing innocent bystanders. Ideally the heroes have SDC weapons on hand for these situations and they can do as they like but may find serious consequences for collateral damage. Borgs and Juicers could probably crush the larger rats with their hands & feet. Lets say for the sake of storytelling no one was allowed to bring their power armor or vehicles into town.
Eventually the group tracks down a secret underground science lab where less than stellar researcher was working on a teleportation device which somehow accidentally became an enlarging device. By either pure accident or nefarious sabotage, the guinea pigs (in this case rats) got loose and starting running thru the englaring device in droves. The PC's must decide how to deal with this situation, keeping in mind the teleporter/enlarger is made from MDC materials but the surrounding lab equipment and the house or basement is most likely not. Also there are many rat cages and the rats seem to be moving regularly thru the device and racing into town in order to "dine", so if the players are inclined to help they won't have time to analyze the machine while catching rats and shutting it off. GM's choice if the power supply is obvious or hidden. If they succeed the town will likely be grateful for help and offer free supplies at a minimum.

Possible Variants: The players are not passing thru a random non-descript town when they see the rats but hear an SOS call on their radio or possibly see scared residents fleeing as they approach.
Instead of rats it could be any small mostly harmless animal that would be easy to acquire in bulk in the wilderness of Rifts North America. Fantasy or mundane. Earthworms would be gross but fun. Or any one of the tiny creatures found in the Conversion Books. Or even a dimensional being that originated in the 3 Galaxies and was somehow rifted to the basement lab in large quantities. Now the players have to decide which to disable first, the rift/portal or the device. Its also possible the rats are not an urgent threat but the villagers lack the tools to deal with the monster vermin properly and also not obliterate their little city. Perhaps as the PC's arrive some nut (possibly the same nut who loosed the animals) is proposing some sort of chemical weapon or another more dangerous predator that will also have to be captured or killed later on. A more interesting variant would be a single moderately dangerous creature that becomes even worse when enlarged, like a Dark Conversion gremlin.

This adventure can be as funny or brutal as the GM sees fit in his/her campaign.

[Shorty Lickens]

Also instead of a walk thru device it could be a handheld or turret gun and either malfunctioned on its own and started shooting rats, or was snatched by the above mentioned rift gremlin who thought it would be funny to blast the little buggers and open the door. Again GM's choice as to whether the bumbling scientist survives this event or not. He may be thankful or hostile depending on his mindset and how much the players destroy trying to solve his problem for him.
And of course you will have to decide how long and complex this process is. They could spend days tracking down the rat source in a mid sized town and delving far into an underground lair with other objects and creatures in their way of the rat machine. Or they could just walk right into the nearest building on the only street, and see it as soon they turn on the lights. A large well developed town will probably have their own law enforcement and/or exterminators. Not to mention residents with a variety of MD or SD weapons and possibly MDC buildings, meaning they would not need help from the group.

[taalismn]

This adventure can be as funny or brutal as the GM sees fit in his/her campaign.

"Rodents of Unusual Size? They're just a my-*LUNGEBITE!*