Penny Lane arrives at a remote Australian beach house with her best friends Toni and Amy to celebrate her university acceptance during the sweltering summer of 1986. The trio expects a private getaway, but the atmosphere curdles when Penny's resentful cousin Kat appears uninvited with insult-laced cupcakes and a hidden agenda. After Kat drugs Penny into unconsciousness and is forced to leave, she returns with her menacing boyfriend Angus and his crew, transforming the sanctuary into a violent occupation. Toni and Amy find themselves isolated and outnumbered as the intruders seize control of the house, forcing the friends to rely on wit and desperation to survive the escalating threat.