The Vidiians experiment on B'Elanna, splitting her into two beings... one fully human, the other fully Klingon.
1-06: Eye of the Needle
Optimism ensues when communication with a Romulan in the Alpha Quadrant may yield a way home.
1-10: State of Flux
When Starfleet technology is discovered aboard a disabled Kazon ship, a search ensues for the betrayer.