A condensed version of the novel had been published earlier in Boys' Life (September, October, November, December 1952) under the title "Tramp Space Ship".

The twin teenage boys, named Castor and Pollux after the half-brothers of classical legend, buy used bicycles to sell on Mars, their first stop, where they run afoul of local regulations, but their grandmother Hazel Stone saves them from jail.

Instead of returning home as originally planned, they decide to travel to the asteroid belt, where the equivalent of a gold rush is in progress, prospecting for "core material" and radioactive ores.

En route, the rapidly maturing, fertile (and pregnant) flat cat and its offspring quickly cause a population explosion aboard, so the family places them in hibernation and later sells them to the miners.

[3] The similarities to the flat cats and some specific story events involving them was brought to the attention of the Star Trek staff when Desilu/Paramount's primary in-house clearance group, Kellam de Forest Research, submitted a report on the script on August 11, 1967, noting the similarities of "a small, featureless, fluffy, purring animal, friendly and loving, that reproduces rapidly when fed, and nearly engulfs a spaceship".

In his authorized biography, Heinlein said he was called by producer Gene Coon about the issue and agreed to waive claim to the "similarity" to his flat cats because he’d just been through one plagiarism lawsuit and did not wish to embroil himself in another.

Conversely, the tribbles copy not only the flat cats' general appearance, behavior, the effects of their purring on people, and the particulars of their reproduction when fed and unfed—but that they are obtained from a merchant who does not warn customers of their dangers, and ultimately unloaded from the ship onto another unsuspecting party.

[4] The screenwriter of the episode, David Gerrold, admits that he had read the Heinlein book years before writing his screenplay but claims he was not consciously aware of the similarities until the Kellam de Forest report.

Jo Walton writes that as a teenager, she considered the book one of the weakest of the Heinlein juveniles; upon rereading it as an adult, it "leaves me feeling I can't get no satisfaction.

Heinlein himself wrote, in a letter to his agent after he had finished the first quarter of the book, that it had "an unsatisfactory story line thereafter," and that he found a "domestic comedy" "harder to write" than "revolutions and blood.