The conflict between Rosalind and Sophie concerns jealousy over David.

Sophie and Rosalind both have a connection to David, and both are jealous.  He can communicate telepathically with Rosalind, and she is the one he is in love with.  David has cared about Sophie since they were kids, but she sees now that she can’t compete with what he has with Rosalind.

'Go on, laugh at me, God damn your lovely face. Laugh at me because I do want him, me !' She gave a queer, choked laugh herself. 'And what's the use? Oh, God, what's the use? If he weren't in love with you, what good would I be to him -- like this?' (Ch. 16)

Sophie comes to understand that David and Rosalind are going to end up together.  She is what she is, and there is nothing she can do about it.  Because of the society she grew up in, she was sterilized and thrown away at a young age for having too many toes.  Then she became something she thinks David would not want. 

As lover to David’s uncle “Spider” (Gordon) and unable to have children, she feels trapped between the two worlds.  When David returns, the presence of Rosalind is like slap in the face to her.  She is a reminder of the life she can no longer have, as well as the boy she loved.

Sophie manages to get past the conflict with Rosalind and be all business.  She knows that things are serious and they are all in danger.

Sophie woke up a few minutes later. She seemed calm, competent again, as though the last night's storm had blown itself out. She sent us to the back of the cave and unhooked the curtain to let the daylight in. (Ch. 16)

Just as they are being attacked, the Sealanders arrive and save David and Rosalind.  In the end, they do get to be together.  However, Sophie is cut down in the attack, as is the Spider.  Only the telepaths are rescued.

This novel is an example of what happens in a society that is intolerant.  David's society cast Sophie out because she was different.  What they did to her was inexcusable and cruel.  They were ignorant and harsh.  David and Rosalind were also treated harshly because they were different, as telepaths.  They always had to live in fear.  This book's ending is not necessarily a happy one, since Sophie is killed, but it is somewhat hopeful, since the telepaths escape.