Around last year, I got the Ark in Space on DVD. I already have it on video. Oddly, most of my 4th Doctor featured Harry and Sarah Jane. This is one of my favorite Harry episodes. In it, Harry is his usual sweet old British self. In the beginning, he talks about almost losing his mind during his first ride on Tardis to Sarah Jane. As the adventure begins, Sarah Jane wonders off while the Doctor and Harry gets constantly zapped by the ship's security system. Harry ended up losing both shoes so he wears dark socks during the entire episode. It is amusing as he called the ship CMO bird while she wondered what he meant. Moreover, he became the acting secondary medical officer on the ship for a brief time. To me, it seemed that he wanted to somewhat stay became the other medical officer. His duty was to the Doctor, Sarah Jane, and the UNIT.
This episode is one of the reasons that they should kept Harry. Yes, it was somewhat awkward character but some of the Doctor's other companions were before they became fully developed. He is one of my favorite companions. Viva Harry.