An all-time rager of a teen house party turns to terror when aliens attack, forcing two warring siblings to band together to survive the night.
Searching for a place to host a Halloween party, teenagers Billy, Dallas, and Trish bully young friends Gary, Jack, and Miles while the kids film a homemade fantasy movie with Gary's older sister Samantha in a barn. Billy only stops harassing the boys when he seemingly takes an interest in Sam, who becomes enamored with Billy. Gary breaks his arm while recording a wrestling scene for the movie. Due to Gary getting injured on her watch, the duo's rich parents ground Sam before leaving the siblings home alone in the movie so she can start pursuing a more adult persona that might impress Billy.
Billy harasses the three friends once more when he comes to see Sam. Secretly wooing her so he can use the house for his party, Billy pretends to appreciate Sam's interests in action figures and wrestling. Sam and Billy start making out on her bed, but are interrupted by the boys when they burst in to harass Billy. The boys are then interrupted by a loud noise and lights outside, which Sam dismisses as another one of Gary's pranks. Sam calls her brother a loser and tells him to "grow up." Billy convinces Sam to host a Halloween party while her parents are away by claiming he wants to show her off as his new girlfriend. While Gary, Jack, and Miles secretly spy on the festivities using a drone camera, Billy and his friends intentionally trash the house. Sam tries to kick out the disrespectful partygoers, which causes Billy to become physically threatening with her.
Gary, Jack, and Miles interrupt the party by hijacking the TV to broadcast a message calling out Sam for changing her personality to appeal to Billy. Gary also shows footage of Billy making fun of Sam to Trish before urinating all over Sam's bedroom, which brings Sam to tears. Billy grabs the boys from their hiding spot and brings them out to be beaten in front of everyone. Alien creatures previously seen attacking three fishermen aboard a boat suddenly burst into the house. Partygoers flee while Gary, Jack, Miles, Billy, Dallas, and Trish are captured. Sam avoids capture by barricading herself in her bedroom.
Once the coast is clear, Sam dons a scuba suit that she uses to swim underwater to the alien spaceship submerged in the lake behind her house. After sneaking into the craft, Sam witnesses the aliens using a slime pit to melt Trish, which Gary later theorizes is how the aliens create fuel for their ship. Another type of slime mutates Dallas into a creature with bladed claws. Armed with an unusual sword she found, Sam slays several aliens to rescue her brother before he can be slimed. With Dallas pursuing him, Billy steals the scuba suit to save himself and return to the surface. Sam still escapes with the three boys by having everyone hold their breath before resurfacing.
Sam, Gary, Jack, and Miles return to the house to discover Billy locked himself safely inside. Dallas still breaks through a window to resume his pursuit of Billy while Sam and the boys escape on their bicycles. In between more alien encounters, Gary and Sam reconcile. Sam, Gary, Jack, and Miles seek shelter in the barn where they had been filming their movie. Billy, who also went to the barn to hide, suddenly takes Sam hostage with her sword. The three boys attack Billy so Sam can get free, but Billy gravely wounds Jack by thrusting the sword through his stomach. While Miles tends to Jack, Dallas's sudden arrival scatters Billy, Sam, and Gary. Dallas ultimately kills Billy before the siblings finally defeat Dallas.
A tractor beam from the alien ship starts pulling Gary into the sky. Acting on a suggestion from Jack, Miles lights a fireworks rig that the friends built for their film's grand finale. The rig gets sucked into the sky too, and its subsequent explosion destroys the ship and saves Gary. With Jack barely clinging to life, Sam, Gary, and Miles start rushing him to safety until more aliens surround them. Armed soldiers from an unknown organization suddenly kill all of the creatures with automatic weapons. Unexpectedly, the soldiers then retrieve the sword, take the kids captive, and transport them to a secret location where they are put in stasis, until another spaceship arrives.
In a post-credits scene, another alien came into Sam and Gary's destroyed house until their parents came, with no knowledge of what really happened. The alien comes from behind and attacks them using the same slime that mutated Dallas. (thanks to Wikipedia)
