David describes two incidents that really affected him and made him look at things differently. One of the incidents was meeting Sophie, because he realized that mutants were not really ugly non-humans. The other event was that he realized that Petra could also think in thought-pictures.
The telepaths are able to avoid detection, and live relatively normal lives for six years.
Until, in fact, the day when we discovered that the eight of us had suddenly become nine. (ch 8)
He was working in the fields one day when he gotten a sudden powerful sensation to come to the river. When he got there, he realized Petra was in the river about to drown. Rosalind also came running from the other direction.
The children were able to say they had heard screaming and that is why they came to the rescue. People believed them. They also did not explain to Petra what had happened, since she had not even realized she was doing it.
Rosalind and David realize that young Petra is much more powerful than any of the others. They are in great danger if they are found out.
We saw no reason then why it should not continue to stand as it did, for all of us; no alternative, indeed. If we did not remain hidden, we should be finished. (ch 9
From this moment on, David has to protect Petra. He just looked at her as a normal little sister before this event. At this point, he begins to really worry about her. She does not realize what she is doing, and she is strong enough to bring the others. They know they can’t gather in one place, because people would be suspicious. If they are found out, they are dead.
The inclusion of Petra in the thought-picture group is an important milestone for David. Before he realized she was also telepathic, he just had to protect himself. Now he is thrown into the role of protecting her.