In this festive adventure, Simon Templar prepares to participate in a Christmas charity event by dressing up as Santa Claus. However, his plans take an unexpected turn when he's mistaken for a criminal named "Fats" Boylan, who is also donning a Santa suit. Despite the nickname, Fats is anything but hefty, leading to a case of mistaken identity. As Templar delves deeper, he uncovers a plot involving stolen jewels hidden within a Santa costume.