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.

Susan Lewis, a young woman scheduled to participate in a beauty contest the next day, goes to bed early after taking a heavy sedative. The next morning, she discovers she is only person left in her village - the entire population of Hambledown has disappeared over night! This case is so strange that the local authorities immediately ask for the Department to take over. As Sir Curtis points out to Stewart, "Your Department is getting quite famous!"

Thus, with virtually nothing to go on, the three investigators travel to the deserted village and set up their HQs in the pub belonging to Susan's father. Stewart and Jason set out to check the surrounding areas, while Annabelle runs her research and finds evidence of some missing paint. Meanwhile, Susan is lured to a doctor's office and given an injection... once the detectives realize this, they rush her to a hospital, where it turns out that the injection was completely harmless - nothing but water.

As if this wasn't strange enough, they also discover a burnt patch of grass, and wonder what was burnt and why... Jason theorizes that if it were known that another person was 'left behind' in the village that night apart from Susan, the perpetrators would have to make a move... so Stewart sets himself up as bait. The plan works, up to a point, but when the gangsters show up, they knock out Jason and force Stewart to 'accompany' them at gunpoint... which leads him much closer to the case's solution, yet also into danger, and ultimately, in Jason's words, to "a marvelous twist at the end!"