“Homefront” is episode eleven of season four of Star Trek: Deep Space Nine.
The wormhole has been opening and closing, but no ships have come through it. Then the station gets a call from Starfleet on Earth. There was an explosion at a conference between Romulan delegates and the Federation, killing 27 people. The explosion was no accident; it was a bomb set off by a Changeling.
Sisko and Odo are called to Earth to help with the Dominion threat. The Federation’s President is reluctant to go to the lengths that an admiral is, so Sisko and Odo have to convince him otherwise. As a consequence, Sisko is put in charge of Starfleet Security as related to the Changelings. It turns out that they have managed to get inside Starfleet Command.
Events on Earth rapidly start getting worse, and there’s a fear that a cloaked Dominion fleet may be on its way.
The story continues in the next episode, “Paradise Lost”.