They were both Caucasian males with short hair; one had light brown hair, while the other had black hair. They wore light gray military uniforms and dark gray helmets with the letters "MP" printed on them in white. They had walkie-talkies attached to their belts.
They behaved in a stern, grumpy, and intimidating manner, warning intruders not to trespass; Lester described them as being "meaner than a bull with a toothache." They also showed a sadistic streak while they were threatening Shaggy Rogers and Scooby-Doo.
They confronted Shaggy, Scooby, Crystal, and Amber when they traveled too far beyond a fence in search of a jackalope. They claimed that the area was under government investigation and ordered them to leave, making Crystal angry. When they left, Crystal revealed to Shaggy that she was a government agent and convinced to him and Scooby go back in with her. They found the cave where Shaggy and Scooby first saw the aliens. The fake MPs came back because they heard something, but they left when Scooby used his tail to fake the presence of a rattlesnake.
The four explored the cave until they discovered gold. However, two of the aliens showed up and chased them. The rest of the gang also found the gold, but they were spotted and chased by the third alien, and the fake MPs eventually joined the chase. Shaggy, Crystal, and the dogs escaped, but the others were captured by the aliens, who were revealed to be Steve, Laura, and Max.
The fake MPs caught up with Shaggy, Scooby, Crystal, and Amber and cornered them. Shaggy and Scooby started acting like they knew martial arts, prompting the fake MPs to laugh at them. Crystal and Amber revealed their true alien forms, scaring the fake MPs away. Shaggy and Scooby didn't see the transformation, so Crystal claimed it was them who scared them off.
The fake MPs ran back to the SALF crew in a panic, telling them that they had seen real aliens, but they didn't believe them. When Shaggy, Crystal, and the dogs came back, the fake MPs figured that what they saw must have been some kind of trick and started threatening Shaggy and Scooby again. Unfortunately for them, Crystal's lie gave them an ego and made them think they could take the fake MPs, forcing her and Amber to reveal their true forms again, frightening both the fake MPs and the SALF crew. Amber took a steel girder and used it to restrain the fake MPs along with Max. The fake MPs were ultimately arrested along with the SALF crew.
Fake MP (brown hair): Bunch of hippies!
Fake MP (brown hair): We came to see if you're alright.