A standard illustration of the time-distorting effects predicted by Einstein's relativity equations is the traveling twin problem, where one twin remains on Earth while the other travels at near-light speed to some distant destination. Heinlein takes this textbook concept and adds two other ingredients to the mix: the twins are telepathic, and they are real people, in concocting this nice blend of great adventure and hard science.
Tom and Pat are the twins in question, targeted by the Long Range Foundation as a potential communications pair on the first exploratory star-ships due to their telepathic ability to communicate over any distance at (truly) instantaneous speeds. Which one will go and which will stay forms the initial conflict of this story, and how the decision is made provides a strong base for filling in the character of each, along with some interesting psychological insights into the problems that face close siblings. While still on Earth, this section also allows Heinlein to throw in some of his typical comments about bureaucracies, government meddling, taxes, population control, and the non-democratic nature of families, all deftly folded into and directly contributing to the story line.
Once the starship takes off, we find something of a more traditional adventure story, as we follow Pat on the starship and his meeting with the duties and responsibilities of ship-board life and the unforeseen hazards that the ship encounters at each of the stars it explores. In the meantime, Tom is rapidly aging on Earth, the link between the two becomes very fragile, and eventually Pat manages to establish a new telepathic link with his niece (and later his grand and great-grand niece). All necessary in order to continue the starship mission, for without being able to report the findings of the explorations, there is little point in continuing. As we move further and further out in time and space, we can see Pat grow as person, melded both by these external events and his own musings on the purpose of life and humanity, and it is this very growth that really provides the best portion of the 'entertainment'.
Heinlein fully recognized that positing instantaneous communications (of any nature) was a violation of Einstein's basic theory, and rather than ignore it, he used it as a springboard to a new science that forms the basis for the ending of this book. It also allowed him to neatly finish off the story line of the two twins, but I found the ending not quite satisfying, a little too pat and quickly done (and with some gender-roles that would be considered decidedly non-politically correct today). Still, this is one of the best of his so-called 'juvenile' novels, both due to its great science and very solid characterization, couched in his typical, unforced American prose, and with enough 'meat' on its bones to engross any reader.