Deep Space Nine's fifth season was a turning point as the Klingon faction, instigated by Worf's arrival, made all efforts at diplomacy moot. In the opening episode a chilling possibility is proposed: have the Changelings infiltrated already and replaced key personnel? Expanding on the new theme of duplication, the crew also made numerous trips to their mirror-universe counterparts. The crew has new uniforms, and the series passes its 100th episode. The popular "Trials and Tribble-ations" puts Sisko and crew in contact with Kirk.