While making a supply run to the front lines, on the barren planet AR-558, Sisko encounters Starfleet troops that have been decimated during their five-month occupation of the largest Dominion communications array in the sector. If they can figure out how it works, Starfleet will be able to tap into Dominion transmissions. In addition to daily skirmishes with the enemy, the shell-shocked forces are constantly threatened by "Houdinis" — anti-personnel mines that "hide" in subspace, then explode without warning. Determined to help commanding officer Nadia Larkin and her troops, Sisko stuns his crew by announcing that they're staying at the site indefinitely.
The situation is grim. Jem'Hadar forces send in holographic soldiers, which draw Starfleet fire, to determine troop strength without risking casualties. Sisko decides to take action. He orders Ezri and Larkin's chief engineer Kellin to devise a way of exposing the invisible Houdinis, and against Quark's wishes, sends Nog, Larkin and a battle-hardened soldier named Reese on a scouting patrol to pinpoint the enemy's location.
Nog is instrumental in discovering the Jem'Hadar camp, where troops outnumber Sisko's forces by three to one. But while heading back to report their findings, Larkin is killed by an enemy phaser bolt and Nog is struck down. Reese rushes the injured Ferengi back to base camp, where Quark later angrily informs Sisko that Nog will lose his leg.
While Nog recuperates at a makeshift infirmary, Ezri and Kellin are able to materialize the Houdinis. Rather than disarm the mines, Sisko has them relocated to the ravine through which the enemy must pass to reach the outpost. The first wave of Jem'Hadar soldiers is destroyed when they try to attack, but soon, a swarm of enemy warriors invades the site.
As the Jem'Hadar charge continues, Starfleet troops engage in hand-to-hand combat to defend their position: Kellin dies while saving Ezri's life; Quark kills a soldier who bursts into Nog's room; and Sisko rescues Bashir from a nearly fatal attack. Later, when the battle is over and fresh reinforcements arrive, allowing Reese and the other survivors to leave, Sisko reflects on the human toll that's been paid to retain control of the outpost, and on the importance of never forgetting the soldiers who have died.