DS9 Season 3 Episode 20 Improbable Cause

Plot

When Garak's shop explodes, Odo finds that it was destroyed by a bomb set up to make it look accidental. He tracks down a possible suspect but before he can catch up with him, the suspect is killed when his ship blows up.
Odo studies the case further and discovers evidence that the Romulans, a rival alien race, are behind it. They probably hired the dead suspect, and then killed him to help bury evidence. Odo digs up more information about the incident and finds that it is much more complicated than an assassination attempt on Garak. It seems the Romulans are planning to invade Cardassia and several members of the Obsidian Order, the Cardassian secret police, have been killed in mysterious mishaps recently.
Odo tries to get Garak to admit that he blew up his own shop in order to get Odo to begin an investigation. All of the Cardassians recently killed were close to Enabran Tain, Garak's Obsidian Order mentor. Now that all of his associates are being targeted, Tain may be in danger. Odo and Garak set off to locate him.
En route they are captured by a Romulan starship. When they are brought aboard they find Tain is there. He tells them he ordered Garak's assassination, as well as those of the rest of his former associates. He is wiping his history clean in preparation for a power play. The Tal Shiar, the Romulan intelligence agency, have formed an alliance with the Obsidian Order, and they are planning an attack on the Founders in the Gamma Quadrant. Once the Dominion is taken out, Tain plans to take control of the Obsidian Order. Since his attempt on Garak's life failed, he asks Garak to join him, and Garak immediately agrees.