The Doctor is a very strange person. Ian has only known them for a few months, but he knows that statement is very much true. The Doctor is... well, odd.
Sometimes, they are rather pleasant to be around. The Doctor makes conversation, actually listens to him, Barbara and Susan, and comes across as fairly sociable, if still a bit blunt and tactless (as always).
But then there are the other times, where the Doctor is not so pleasant to spend time with. Ian has already recognised the pattern, and knows when the Doctor’s mood is changing. They start to shut themself off, their voice becoming flatter as they become more tense and snappy, their hands constantly fiddling with the lapels of their blazer. The Doctor often ends up snapping at Ian or Barbara (or even Susan), before storming out of the room.
What is remarkable is how quickly this change can occur. It only takes a loud noise or a TARDIS malfunction for the Doctor to start shutting down. Other times, they gradually seem to wear down over the course of the day, lasting until the evening before getting tense and irritable.
And the most confusing thing is that he has no idea why this happens.
One day, after the Doctor shouted at him and stormed out of the room, Susan hurries after them, as she always does. When she returns, Susan gets Ian and Barbara to sit down.
“Grandfather wants me to explain something to you,” she says, and then she tells them that the Doctor has a disability called autism.
With a great deal of knowledge, Susan tells them all about autism, about its traits and the myths surrounding it. And, as he listens, everything begins to make sense. The Doctor isn’t odd; they’re autistic. He understands now.