our juvenile hero, his companion and their mentor enter an audience room so vast that its ceiling is not visible;
there are machines, no two alike, standing along the walls;
the three Earthmen must walk to where the Hegemon waits;
he is blue, humanoid, over eight feet tall and simply clad in black with bare arms and legs (see image of book cover);
guided both by ancient customs and by machines, he simply states that the Hegemony will forcibly incorporate humanity as a subject race.
On reflection, I realised that I would have accepted without question such an account of an alien contact when I was at primary school. Identifying with the young hero while imagining the companionship of both a peer and an elder, I would expect a hostile alien to be humanoid but menacingly large, also that his civilization would have the, for us, paradoxical features of ancientness, as represented by large, old buildings, and of high technology, as represented in this case by "machines" simply placed along the walls of the vast chamber.
This reads like an archetypal account of human-alien contact. Another such is Dolph Haertel's meeting with the last surviving member of the dominant race of Mars under Lacus Solis in Welcome To Mars.