David Starr, Space Ranger introduces the series' setting and main characters. The novel is set around A.D. 7,000 (five thousand years after the first nuclear bomb, as stated at the beginning), when humanity has founded colonies on the inner planets of the Solar System, as well as spreading to other planetary systems with separate and sometimes hostile governments. The most powerful organization in the Solar System is the Council of Science, which uses scientific expertise and intrepid field agents to counter political and military threats to Earth's government.
Protagonist David Starr is a young biophysicist orphaned as an infant and raised by his guardians Augustus Henree and Hector Conway, high-ranking Council members who send David on his missions for the Council. They tell him of some 200 recent victims fatally poisoned by produce imported from Mars. Fearing a conspiracy to start a food panic and wreck interplanetary trade, they send Starr undercover to Mars.
There Starr meets John "Bigman" Jones, a short but pugnacious Martian farmboy blacklisted by the Martian Farming Syndicates after witnessing underhanded dealings. When his former boss Hennes orders Bigman out of the Farm Employment Building, Starr stands up for him and gains positions for both himself and Bigman. The humiliated Hennes subsequently has Starr and Bigman knocked out and brought to the farm owned by Hennes' boss, Mr. Makian, who apologizes and appears friendly.
Starr says his name is Williams and he came from Earth to investigate his sister's death from food poisoning. Makian sends him to the farm's agronomist Benson, who explains that the poisoned food came from several Martian farms and was exported through the domed Wingrad City. Meanwhile Makian and other farm owners have been pressured to sell their farms for ridiculously small sums. Benson also suggests that intelligent native Martians living below the planet's surface are poisoning the food to drive off the human colonists.
Makian sends Starr and Bigman on a survey of the farmlands led by Hennes' friend Griswold. Unfamiliar with Martian gravity, Starr nearly skids his sand-car into a crevasse. After Bigman discovers the car is missing the required ballast-weights, Starr accuses Griswold of trying to kill him. They fight, and Griswold falls into the crevasse.
The next day, Benson makes Starr his assistant, to keep him from the vengeful Hennes. Bigman receives his references from Hennes and takes his leave, but returns that night to the farm to meet Starr, and tells him he has recognized him as belonging to the Council (whose members are publicly listed, though they try to minimize publicity).
Starr tells Bigman that he believes in Benson's Martians, and that the crevasse into which Griswold fell is an entrance to their caverns. Starr descends into the crevasse and is captured by Martian aliens — disembodied intelligences curious about the Earthmen on the surface. They know nothing of the poisoned food – although, according to them, any organic matter of Martian origin is poison for Earthmen. They give Starr the name "Space Ranger", and present him with an immaterial mask producing a personal force field that can protect and disguise him.
Starr uses the mask to shield himself from a Martian dust storm as he returns to Makian's farm, where he is questioned on how he survived the storm and answers that he was rescued by a masked man called the Space Ranger. Benson tells him the farm owners have received an extortion letter from the poisoner, threatening to a thousandfold increase of poisoned food unless they sell out to him within 36 hours.
After again trying to kill Starr, Hennes accuses Starr of being the poisoner. Bigman enters, having brought from the city Dr. Silvers of the Council, who announces that the government has declared a System Emergency and that the Council will take control of all farms on Mars. If the mystery is not solved by the deadline, all Martian food exports to Earth will stop, forcing food rationing.
Disguised by his Martian mask, Starr confronts Hennes, who blinds himself firing a blaster at the protective field. Starr searches Hennes and finds incriminating evidence. The next day, at a meeting with Silvers, Makian, Hennes and Benson, Starr again appears as the masked Space Ranger. He reveals that it was Benson who poisoned the food while pretending to take samples of it, while Hennes was his accomplice in contact with criminal syndicates in the Asteroid Belt; the distraught Benson confesses. Afterward, Bigman confides to Starr that despite the disguise, he recognized him by his black-and-white boots, too plain for any Martian.
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