DS9 Season 4 Summary . Gowron orders a Klingon invasion of the Cardassian Union . When the Federation condemns the invasion , and Sisko protects the members of the Detapa Council , Gowron withdraws from the Khitomer Accords and attacks the station . Although the attack is unsuccessful , the Klingons refuse to give up the Cardassian colonies already seized . Consequently , Captain Benjamin Sisko asks Worf to join the crew of Deep Space 9 . Julian Bashir attempts to find a way to remove the Jem ' Hadar ' s addiction to ketracel white . He clashes with his friend , Miles O ' Brien , by doing so , and is eventually unsuccessful . Kira Nerys and Dukat embark on a personal journey to find the Cardassian transport ship " Ravinok " that disappeared six years previously . Kira is searching for a friend ; Dukat is searching for his daughter , Tora Ziyal . He intends to kill her to protect his family , but once he finds her he is unable to do so and she returns with him to Cardassia with devastating consequences to Dukat . Jadzia Dax is reunited with Lenara Kahn , the wife of her former host . After spending time together , their feelings resurface , but Kahn decides that she cannot risk getting involved with Jadzia . Nog leaves for Starfleet Academy on Earth , though a shuttle accident en route temporarily strands him , Quark , Rom , and Odo in 1947 . Kor , along with Dax and Worf , discover the long - lost Sword of Kahless in the Gamma Quadrant . They conclude that the Klingon Empire is not yet ready for such a powerful symbol and leave it adrift in open space . After it is discovered that at least one Changeling infiltrator is active on Earth , Starfleet makes Benjamin Sisko acting head of Starfleet security on Earth . After a power outage , Admiral Leyton manages to convince the President to let Starfleet declare martial law . However , Sisko soon discovers that the power outage was actually caused by Red Squad , Leyton is arrested for treason and Starfleet forces withdraw . First Minister Shakaar Edon begins a relationship with Major Kira , which forces Odo to deal with his feelings for her . Kira later helps Dukat capture a Klingon Bird - of - Prey and convinces him to let Tora Ziyal live aboard Deep Space 9 . Worf is reunited with his brother Kurn who has been banished from the Empire due to Worf ' s condemnation of Gowron ' s invasion of Cardassia . Worf decides to erase Kurn ' s memories and let him become a member of another house . Meanwhile , Benjamin Sisko must fight for his role as Emissary when a Bajoran , Akorem Laan arrives at the station of 200 years in the past claiming to be the Emissary . The challenge is taken to the wormhole aliens who return Akorem to his time , letting Sisko resume the role . In the mirror universe , the Terran Rebellion captures Terok Nor from the Klingon - Cardassian Alliance . With the help of the Sisko from the primary universe and the rebellion ' s new ship , the , they successfully repel an Alliance attack , though the mirror is killed during the incident . Sisko ' s girlfriend Kasidy Yates is suspected by Odo and Eddington to be a Maquis smuggler . This turns out to be true , and Kasidy is sentenced to six months in prison , however Eddington is also revealed to be a member of the Maquis and steals twelve industrial replicators intended for the Cardassians . The crew of the helps a group of Jem ' Hadar led by Weyoun eliminate another group of renegade Jem ' Hadar . After an accident on the runabout , Keiko O ' Brien is injured and her baby must be transplanted into Kira ' s womb in order to survive . Quark is forced by Brunt to break a contract , leading to the revocation of Quark ' s business license . Odo is infected with a morphogenic virus by the Founders , which forces him to return home to face charges for killing another Changeling . As punishment , he is made a human . During the mission , Garak makes an unsuccessful attempt to take control of the " Defiant " s weapons to destroy the Founders , leading to his incarceration for six months . Upon Odo ' s return , as tensions continue to escalate between the Federation and the Klingon Empire , Odo realizes that Gowron has been replaced by a Changeling . Dennis McCarthy ( credit only appears in episodes not composed by McCarthy ) Robert della Santina