The Doctor's original companion, Susan, left the series early on in the second season, the first indication that the cast wasn't as fixed as it might have appeared from watching the first season alone. Unlike many other stories of the time, Susan's last episode doesn't end with the first few seconds of the one following it, giving a possible slot for continuity-minded GMs to insert stories with only Ian and Barbara as the companions. (On the flip side, the fact the Doctor temporarily forgets about Susan's absence at the start of 'The Powerful Enemy' suggests that it can't be a very long gap, if it exists at all).
At any rate, the very next episode introduces a new companion for the first time since the show started.
It's hard to come up with a role-playing analogy for what's going on here. That's because, having decided, for whatever reason, to ditch the Susan character, her player comes up with a new PC that's essentially the same idea. Perhaps she likes the character concept, but wants the chance to smooth off a few rough edges with a second attempt.
