Before you watch this, though, it's best to know that it was written, directed and produced by a high school student and shot in Super 8mm film. That said, this is a very ambitious and creative movie that has the vibe of a Frank Henenlotter movie.
A Venus probe crashes back to Earth and lands in a pond, where to kids find the parts floating around. One of the pieces of equipment has "Danger: Radioactive Materials" handwritten on it with a magic marker. One of the kids is bitten by a weasel, presumably (we never see an actual weasel) which escapes down a hole. So they decide to pour the radioactive contents down there. This quickly mutates the weasel into this paper mache' alligator looking thing. It chomps up the kids. Then, when it is running across a nearby road, it gets hit by a car and loses an arm. This arm is larger than a human appendage. The driver picks it up and takes it home where he ends up showing it to a friend.
It turns out that the weasel has rabies. We find this out when a very young looking scientist, who looks like one of WELCOME BACK KOTTER'S sweathogs, explains that he made the probe crash because he wanted the samples from Venus. He is telling this to two guys before he drugs them and uses them for an experiment. He wants to transfuse the rabies out of the Alligator-Weasel monster and uses them to do it. One of the guys mutates and begins to resemble a giant potato and ends up fighting the big mutant weasel head to death. The other guy escapes.
With all the remakes going on this past decade, this is a movie someone needs to remake and give a special effects uplift. It is such a weird idea.