While salvaging components from DS9's sister station, Empok Nor, the crew run afoul of a disturbing surprise left behind by the Cardassians.

With continuing convoys of Jem'Hadar arriving in the Alpha Quadrant, Sisko decides to mine the wormhole to prevent further troop movement. However, this decision forces the Federation into a direct confrontation with the Dominion, and Gul Dukat responds by launching a massive assault fleet against DS9.