An explosion at a Federation/Romulan diplomatic conference on Earth has killed 27 people. Most disturbing is evidence that the bomb was set by a Changeling — meaning the Dominion has reached Earth. Sisko and Odo are summoned to San Francisco to advise Admiral Leyton, the head of Starfleet Operations, on the Dominion. Once there, Leyton informs them that, effective immediately, Sisko will be in charge of Starfleet Security on Earth.
A trip to Earth also means a visit home for Sisko and his son, Jake. They head to the New Orleans restaurant owned and operated by Sisko's father, Joseph. Later, Sisko and Leyton meet with Federation President Jaresh-Inyo, who balks at Sisko's ideas for increasing security. But when Sisko's briefcase suddenly morphs into Odo, Jaresh-Inyo sees the seriousness of the Dominion threat, and agrees to allow Sisko to implement blood screening measures on Starfleet officers and high-ranking Federation officials.
On the grounds of Starfleet Headquarters, Odo encounters Leyton and his executive officer, Commander Benteen. During their conversation, Odo suddenly grabs Leyton's hand, which then dissolves into gelatinous goo. Revealed to be a Changeling, the duplicate morphs into a bird and flies away. The real Leyton is extremely upset by this news, but his discussion with Sisko is interrupted by an urgent call from Jake — Joseph has been arrested.
Sisko returns to New Orleans, where his father has refused to submit to the blood testing now required of all relatives of Starfleet members. Joseph angrily retreats to his restaurant kitchen to chop vegetables, but cuts his finger. Sisko can't help checking the knife to make sure the blood doesn't become Changeling goo. That night, Earth's entire power-relay system goes off-line. When Leyton blames sabotage, Sisko surmises the Changelings have left Earth defenseless.
Sisko and Leyton use the systems of an orbiting starship to transport to the President's office, and convince him to declare a State of Emergency on Earth. Armed Starfleet troops are quickly mobilized across the planet. Sisko prepares his homeworld for war with the Dominion.