When Meg Oliver and her brother, Chris, were on their way to a Halloween party on October 30th, they find Bim stuck in a hedge. After he's pulled out, Meg and Chris continue on their path to the party. Some time later, while a friend of Meg was telling a story about a headless ghost, a brown package smashes through the window. Meg is reluctant about opening it, but Kelly opened the package anyway. A foul stench came out of it, forcing everyone out of Kelly's house and ruining her party.
Back Home, Meg and Chris enter Meg's bedroom to find Bim in it. Bim tells them the package was an aroma message he sent to make them come home, because he felt loney. He introduces himself to them and says that he came from Weirdo Planet. Meg and Chris don't believe him and tell him to go home but he states that he now belongs to them because they pulled him out of the hedge. Meg picks him up and tries throwing him out but he rapidly gains weight forcing her to put him down. This causes Meg to lose it and start tugging at Bim's chin. That when Chris and her realize that Bim really is a alien.
Chris ask Bim why he came to Earth, but he can't remember. As he tries to remember, Bim barfs out a dead bird on Meg's bed. When Chris and Meg's old babysitter, Penny looks into the room and sees the dead bird and Bim, Meg tells her that the dead bird is a joke and that Bim was a friend named Max in costume. Bim offered both Meg and Chris a taste of the dead bird, but they refuse so he shallows it whole. He tells them that he only eats living meat because it tasted fresh and that he heaves it up before it stays down.
Meg and Chris once again tell Bim to leave which made him angry, causing him to drool, his fangs to slid down over his chin, his chest to swell, his face darkened to purple and made him pant and snarl like a fierce animal. As his face brightens to tomato red, Bim tells Meg and Chris that he shouldn't be angered. He then takes a 1930's doll from Meg's antique doll collection and bites its head off then chews its body up, reducing it to nothing but wooden shavings. Meg and Chris tell Bim they would be glad to keep him happy, causing him to calm down and return to normal. Bim then took off his T-shirt and orders Meg and Chris to rub his back for a hour or two. They're at first in defiance, but then Bim attempts to chew up the only 1890's doll in Meg's collection so they give in and rub his back (which made the skin on his back turn yellow and the bumps on it appear to be moving). They rub Bim's back until he gets tired. Bim then clears the bottom shelf of Meg's bookcase of books, climbs onto it and falls asleep.
On the morning of October 31st, Meg wakes up to find her bookcase's bottom shelf empty. She searched for Bim in her bedroom and the kitchen, but he was nowhere to found. It seem like he was gone for good until Meg went over to Kelly's house and hear her screaming. Meg burst into Kelly's bedroom to see her staring at a chewed-up rabbit corpse on her pillow. Upon seeing this, Meg knew this had Bim's name written all over it. After helping Kelly dispose of the rabbit corpse, Meg bumps into Carlos. Knowing that he's into sci-fi and fantasy, Meg tells Carlos all about Bim and what he has done which ends with Carlos not believing her. She takes Carlos to her bedroom to prove herself and they both see Bim eating a squirrel on Meg's bed. Carlos is so sicken by the sight, that he runs away. Bim force feed Meg some flesh from a squirrel rib then eats the last of it, himself. He tells Meg that the chewed-up rabbit corpse he left on Kelly's pillow was a gift for her. This enrages Meg and has her pounding Bim's chest with both fists while telling him to leave, but then Meg's fists got stuck to his damp, sticky skin. Meg struggles to free her fists but manages take them off Bim's chest, only to discover that her hands were covered in big ugly warts.
Bim got mad and smashed the lightbulb from Meg's lamp then tosses the shattered glass across her bedroom. She see Chris and takes to his bedroom, where she tells her that they have get rid of Bim. Knowing that he's been to Earth before, Chris comes up with a plan to take Bim around town to get him confused before dumping him at a Halloween party. Meg tells Chris his plan is no good and heads for her bedroom, then she sees Bim had left through one of two windows. She heads towards it to close it, but sees that he also chewed up all the antique dolls in her collection into sawdust. After dinner, Meg calls Carlos asking him how she and Chris can dispose of Bim. After, a few lousy suggestions, Meg hangs up and sees Bim enter her room through the window. Bim takes off his T-shirt and tell Meg to rub his back. Meg noticed her hands' warts had faded away and wonders if they'll come back if she touches Bim's skin again or worse, cover her whole body. Either way, she didn't have a choice so she rub his back. Bim tells Meg that he enjoy living her and Chris so much, that invited his friends to live with them, too. Shocked by this, Meg runs into Chris' room and tells him she's willing to go forward with his plan.
With Bim following them, Meg and Chris take a bus downtown then another back to their neighborhood. They walked in one direction for blocks then circled back two or three times then headed into Madison Park, crossing back and forth through its paths. Afterwards, they took the Metro across town and zigzagged down several streets until they finally reach a big Halloween party at a enormous house. While Bim was distracted, Chris and Meg took off and caught a bus back to their neighborhood where they run into Carlos and Kelly. Meg, Chris, Carlos and Kelly go to Chris and Meg's house where Kelly finds out about. It seem like Chris' plan worked perfectly until all four of them enter Meg's room and saw Bim standing next to Meg's computer, looking angry.
Bim points out that he found his way back by using his Weirdo Tracking. He got so mad, that he grew to twice his size, his eyes bulged to the size of ping-pong balls, his skin darkened to red, and his fangs slid down from his gums. He smashes his head through Meg's computer montior, rammed it into a window smashing a hole in it, made a huge hole in the plaster of one wall and broke the mirror of Meg's dresser. When Penny come to Meg's room asking what with all the ruckus she sees Bim smash Meg's other bedroom window and starts putting him down which causes him to shrink. Soon, Meg, Chris, Kelly and Carlos joined in, delivering insult after insult making Bim shrink to the point when he was nothing but a tiny ball on the carpet when he rolled himself up. This made him really mad.
Bim unfolds himself, his shoulders popped wide, his chest swelled, his skin darkened to red, his fangs slid down, his eyes bulged like glowing hot rocks and grew to be so big, that his head almost hit the celling. Meg takes Penny to Chris' room and told them to lock the door. Meg, Chris, Kelly and Carlos run out of the house to see Bim (now at least ten feet tall) burst out of it, ripped a tree out of the ground throw it at them. He trips over a bush when running after them, but picks himself and rips it from the ground. Chris, Kelly, Carlos and Meg run towards the backyard, but Bim grabs Meg by her waist with plans to eat her.
Carlos picks up Bim causing him to let go of Meg, but Bim quickly gains weight crushing Carlos. Meg, Kelly and Chris roll Bim off of Carlos and took him into the house while Bim struggle to pick himself due to his weight. Meg, Chris, Kelly and Carlos head to Meg's room and lock the door while Meg calls 911. However, her phone is nowhere to found. Suddenly, Bim burst through the door and knock Meg's dresser over. Meg, Chris, Carlos and Kelly start throwing stuff at Bim, but when Meg throws the Floig at him, he returns to normal.
Bim had finally remember that the Floig was the reason he came to Earth and that the trip made it forget. Now reunited with his favorite toy, Bim starts to feel homesick so he returns to Weirdo Planet. A few seconds later, six Weirdos indentical to Bim come in Meg's room through a broken window saying they've got Bim's invitation to move in. But when Chris and Meg tell them Bim went home, they all return to Weirdo Planet to try and catch up with Bim.
Bim is about three feet tall, has damp, hot, lumpy, sticky and shiny orange skin, a bald head with two snail-like antennae on top, a scrawny neck, a baby face with two tiny, black rasin-like eyes, a crooked smile, a round chin, three rows of straight yellow fangs, a fat pink tongue, little arms with three-fingered hands and a slender chest. He wears a blue T-shirt over a big diaper along with shiny red shoes.