In the novel N or M?, Dr Curtis is a resident of Leahampton, and an acquaintance of Tommy and Commander Haydock.
Dr Curtis and Mr Walters pass Commander Haydock's house on the way back from a walk on the headlands. Tommy walks with them to the gate of Sans Souci, and they converse with him about various local matters.
After Tommy disappears, Dr Curtis tells the police that Tommy had walked with him and Mr Walters as far as the gate of Sans Souci, had said goodbye, and had turned into the drive. That was the last time Tommy had been seen before he disappeared.