I always felt the key strands in Crusade were the Barbara & Doctor stuff - Ian's story isn't really important at all. Barbara may be on her own but her storyline is very well done. Episode 3 contains some of the best performances in the series with Jackie Hill, Julian Glover & Jean Marsh all on top form.
As Christopher says though it does fall apart slightly as the regulars really can't have any significant impact on what goes on. That can often be a problem with the historicals but seems more obvious here I feel.
That said I still love this story.