For me, without a doubt, the most disturbing scene in this novel comes in Chapter 12, which is when Katherine is broken by the torturers who have captured her and her sister, Sally, and Sally is left to confess to the other telepaths that they now know about the group which had through necessity kept itself secret. Note how this is revealed:
...Katherine's barely conscious. She can't send to you. But they keep on asking us both questions, on and on... If you could see what they've done to her... Oh, Katherine, darling... Her feet, Michael--oh, her poor, poor feet...
It is the uncertain description of what precisely has happened to Katherine, only referencing her feet, that leaves the exact nature of the torture she has suffered to the imagination. This makes it much more terrifying and shocking. However, what is much more important, is that the fate that Katherine suffers represents what the Waknukians will do to David, Rosalind and also to little Petra if they catch them. Considering at this point in the novel they are trying to escape, this adds great tension to the plot. The breaking of Katherine is therefore most shocking because of the way it portrays the Waknukians of being willing to torture even harmless and helpless girls to satisfy their notion of what a pure human is and looks like.