As Dr. Bashir and Garak sit to have lunch, Bashir reveals to his companion that today is his 30th birthday. He seems somewhat hesitant to celebrate it since he feels that turning 30 is the unofficial end of youth and the beginning of the slow march into middle age.
The pair are approached by Quark and his alien accomplice, a Lethian named Altovar. Quark rather hesitantly informs Dr. Bashir that Altovar is interested in purchasing a quantity of bio-mimetic gel and is willing to pay any price. Bashir informs Quark that the gel is strictly controlled by Starfleet and is not for sale at any price. To even ask is considered an offense under Federation law. Thwarted, the Lethian left in a foul mood.
Upon returning to the infirmary, Dr. Bashir found Altovar breaking into his supplies in an attempt to steal the gel he so badly wanted. Realizing he had been discovered, Altovar attacked Bashir with a telepathic attack that members of his species are capable of.
Upon regaining consciousness, Dr. Bashir finds his infirmary in shambles. When he calls for help, there is no response from anyone. When he staggers out onto the promenade, he sees it isn't just the infirmary that had been ravaged. The entire station appears to have been attacked from within. Just as it seemed as though he were the only person left on the station, Dr. Bashir finds Garak amid some wreckage. Garak informs Bashir that the Lethian had run rampant throughout the station and was killing anyone who got in his way.
Not willing to believe that everyone but them had been killed, they set out to find help. Bashir was certain that he had heard whispering voices off in the distance and he was determined to find them. As they set out, Garak noticed that the doctor seemed to be noticeably older as they continued. His hair was beginning to streak with grey and his face began to wrinkle. Ignoring the comment, Bashir presses on.
In the ward room, Bashir and Garak finds several of his colleagues holed up in a defensive position, trying to come up with a plan of action to deal with the Lethian, but they argue with each other too much to accomplish anything. Bashir suggests to Chief O'Brien that if he can get the communications system operational, they would be able to call for assistance. O'Brien does so, but only manages to get an audio-only transmission from somewhere outside the station. When they put in on speakers, they hear a conversation between Commander Sisko and Lt. Dax discussing Dr. Bashir's condition. They say that he is in a coma and that his vital signs are getting weaker. Confused at this, Dr. Bashir pulled out his medical tricorder and took a reading. His equipment showed nothing but delta waves. He then realized what has been happening: he was in a coma. The station represented his mind (which the Lethian was trying to destroy) and the people he has been talking to are aspects of his own personality.
Armed with this information, he comes up with a new plan of action. He decides that in order to repair the station and thereby save his own life, he must try to make it to Ops and regain control of the station. Unfortunately, Bashir seems to be aging by leaps and bounds, making the chances of success dwindle as he becomes more and more feeble. In addition, the very people who might be able to help him (Sisko, who represents Bashir's skill and professionalism, Kira, who is his aggression and Dax, who is his sense of confidence) are being systematically killed off by the Lethian. Little by little, Dr. Bashir was being stripped of the means he needs to survive this attack.
As his list of possible allies dwindles and his physical age approaches 100, Dr. Bashir finally makes it to Ops with only Garak left at his side. However, his attempts to repair the station fail and he is left with few options. As he begins to rethink his strategy, Dr. Bashir has a revelation. With all of his other friends and colleagues dead, why is Garak the only one left? Why wasn't he killed as well? Most importantly, what part of his psyche is Garak supposed to represent? The truth dawns on him: Garak is no part of himself. He is the Lethian who has been trying to kill him. Bashir also realized that in the real world, his life isn't focused in Ops, but in the Infirmary.
Still trying to convince the doctor to give up in his futile struggle to survive this experience, the Lethian continues to torment Dr. Bashir. However, he realizes now that the Lethian could not simply kill him outright. If that were the case, he would have done it at any time. He needed Dr. Bashir to surrender. If he were to give up and stop fighting to live, the Lethian would win and Julian would die. Having discovered his secret, Dr. Bashir contained the Lethian in a medical quarantine field and then sterilized the contents, destroying the Lethian in his mind. With the Lethian defeated, Dr. Bashir came out of his coma and realized that he had won.