Valhalla

An alien ship comes through the wormhole hot, its entire crew already dead, carrying Gamma Quadrant technology valuable enough that Cardassia is willing to risk reoccupying Bajor to get it. Sisko has to keep the prize out of Cardassian hands while Kira insists it belongs to Bajor by right, and neither argument matters once the derelict starts taking over Deep Space Nine's own systems.