Visiting the Doctor's house, Sarah-Jane Smith notes an unusual and unpleasant odour, then a sign warning of an experiment with unstable chemicals being conducted within. The Doctor is analysing "a remarkable new petrol additive" which will "improve mileage by some fifty per cent, if the inventor's claims are proved true."
The Doctor and Sarah-Jane test the fuel by taking Bessie for a drive. The additive's inventor is Sir William Dakins, chemist and physicist, who Sarah-Jane reveals disappeared the day before — kidnapped or possibly "defected to the other side". The Doctor doubts Dakins would "hand over an important chemico-discovery [...] to foreign masters", but, after a phone call with the Brigadier, learns to his annoyance that the authorities believe the scientist has indeed defected and left Britain.
Visiting Dakin's sixteenth-century farmhouse, the Doctor takes a small grey metal box with two dials and four knobs, about the size of a transistor radio. This device emits noises which lead the Doctor and Sarah-Jane, in Bessie, to a disused airfield. Sarah suggests calling the Brigadier, so the Doctor tells her to wait and watch while he drives to the nearest village to find a telephone.
Sarah-Jane is captured by three men, armed with automatic weapons, who speak amongst themselves a language she does not recognise. The Doctor returns on foot through woodland, having called for a UNIT helicopter, due in twenty minutes. Seeing Sarah-Jane in trouble, he dances towards the Nissen hut where she is held, picking flowers as he approaches.
Sarah-Jane informs her captors that this is "Doctor Daft" and that "he's simple, nutty ... er .... dotty. Oh, not all there." The Doctor puts on a slow country drawl and tells the men to let them go or he will call the police. The men tell the Doctor to shut up.
Sarah-Jane notices one of the foreign agents nodding off, before she too loses consciousness. She is woken by the Brigadier who says, "Come on, old girl. The party's over." (The foreign agents are not mentioned again.)
The Doctor explains he concealed the metal box in his posy of wild flowers. Sir William Dakins, now freed, reveals that the device is GUARD — standing for Guided Ultra-frequency Anti-terrorist Radio Defence — which he developed (with the Doctor's help) to stop airline hijacking by emitting zorin knock-out gas. GUARD also has a range-finder which homed in on Dakin's wristwatch, thus enablng the Doctor to locate the airfield where the kidnapped scientist was held.
The Doctor scolds Sir William Dakins for inventing the fuel additive, as its use would add to traffic congestion.
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- This three-page text story, illustrated with publicity photos from Robot, was the only Doctor Who content in the TV Comic Holiday Special 1975.
- An editor's introduction to the Holiday Special said, "An exciting Doctor Who story is to be found on page 15 for those who like reading."
- No author was credited for this story.
- Sarah-Jane was spelt with a hyphen throughout this story.
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