Deep Space Nine's fifth series was a turning point from which there was no going back. Character and information overload took over, and the complicated twists and turns in the build up to war either hooked viewers securely, or sent them away with a headache. The Klingon faction instigated by Worf's arrival was occasionally played for laughs, but mostly their hard-headed personalities made all efforts at diplomacy moot. In the opening episode a chilling possibility is proposed as to why might be: have the Changelings infiltrated already and replaced key personnel? Some fans saw this as a flawed X-Files-style development. Nevertheless it sowed a seed of insidious suspicion from here on, affecting all the principal casts' relationship with one another, even allowing Odo and Quark an opportunity to confess a degree of friendship. Expanding on the new theme of duplication, the crew also made numerous trips to their Mirror Universe counterparts.