DS9 Season 7 Episode 23 Extreme Measures

Plot

The Changeling infection has taken its toll on Odo; Bashir estimates he has no more than two weeks to live. Odo asks his lover, Colonel Kira Nerys, to return to her mission aiding the Cardassian rebellion against Dominion rule, rather than stay to watch him die.
Bashir falsely announces that he has discovered a cure, aiming to lure a Section 31 agent to the station. Agent Sloan takes the bait, and Bashir captures him and prepares to interrogate him with a memory-scanning device to learn the cure for the Changeling disease. When Sloan sees no way to get out of Bashir's interrogation, he kills himself. Bashir and O'Brien construct a device that will allow them to infiltrate Sloan's dying mind. They only have a short time to find the information they seek; when Sloan's brain shuts down, they'll die with him if they're still connected.
Upon entering Sloan's mind, they are greeted by a personification of his subconscious. This Sloan apologizes to his family for choosing Section 31 over them, and is about to give Bashir the cure when it is killed by a personification of Sloan's loyalty to Section 31. Bashir and O'Brien are seemingly shot by a figure defending Sloan's secrets, and discuss their friendship before attempting to continue the search.
They seemingly awaken back in Bashir's lab, their mission a failure. In fact, they are still inside Sloan's mind, as Bashir realizes when he sits down in his quarters to read: his book ends where he stopped reading the previous night, and then starts over from the beginning on the next page. Realize they have fallen for a stalling tactic, the two resume the search.
They find Sloan in a representation of his office, which is filled with secret data about Section 31, including the formula for the cure for the Changeling disease. Sloan taunts Bashir with the information in his mind – information that would allow Bashir to dismantle Section 31 once and for all. Bashir is tempted, and begins to read. With seconds left before Sloan's brain shuts down, O'Brien recognizes the trap and tells Bashir to get them out of Sloan's mind. Having obtained the chemical formula for the cure, Bashir administers the drug to Odo, curing him.