Fun Size is a teen comedy centered on a high school senior, Wren (Victoria Justice), who is eager to distance herself from her dysfunctional family by going off to college. Before that can happen, Wren's mother Joy insists that she watch her little brother Albert on Halloween night so Joy can go to a rager with her much younger boyfriend. When Wren gets distracted by an invitation to the party of the year, Albert disappears into a sea of trick-or-treaters. Frantic to locate him before their mother discovers he's missing, Wren enlists the help of her sarcastic best friend April as well as Peng, an aspiring ladies man and co-captain of the debate team, and Peng's best friend, Roosevelt, a sweet nerd whose crush on Wren clouds his better judgment. This unlikely foursome embarks on a high-stakes, all-night adventure to find Albert, crossing paths with outrageous characters every step of the way.