Department S

A tanker is involved in a car crash, and the driver is killed. The other driver discovers the tanker wasn't carrying fuel, but instead its interior contains a large living space, and he swears he saw a young woman inside - but she has disappeared by the time the police arrive. The authorities don't know what to make of the incident, and the Department is called in.

A lighter discovered at the scene of he accident leads them to the woman - and to the discovery that she is involved in a heist in the planning, in which the crashed tanker was also supposed to play a crucial part. The detectives follow her to the South of France, where Stewart poses as a crook and wins her trust after coming out on top after a fist fight with her boy friend, whom they also suspect to be involved in the crime. When the gangster hijack another tanker to replace the one lost in the crash, the Department must hurry to prevent the heist, which is about to go down very soon.