With everyone having a Sunday off - something recommended by psychologist Dr. Heightmeyer - all of the members of the Atlantis team pursue leisure activities. Rodney McKay and Carson Beckett are scheduled to go fishing; Sheppard decides to teach Ronon Dex about golf; Teyla spends some time in the gym; and Elizabeth Weir has a lunch date with the handsome Mike Branton. Their day off however is interrupted by a powerful explosion near the mess hall killing and injuring several members of the team. McKay thinks he knows the cause of the problem - a device found by two junior members of his team may have turned some of them into walking time bombs by creating explosive tumors in them.

With a damaged ZPM, and diminished hopes of survival, the Atlantis team finds themselves stranded in the middle of empty space with hardly any time before they run out of power completely. Dr. McKay is challenged to the max as he tries to find a way to save Atlantis from its doom. Meanwhile, Dr. Keller races to treat Weir's increasing head injuries, but the only answer she can come up with could have grave results for everyone on board.

While exploring a new planet, Sheppard started to watch a strange crystal. He doesn't know why, but he had to touch it. Back on Atlantis, everybody but him started to have some bad nightmares.

Rodney McKay awakens in his lab, tied to a chair and with no memory of who or where he is. He did seemingly leave himself a video message telling him to look for Teyla. The problem began some 14 hours before when Dr. Katie Brown returned to Atlantis with plants from a planet she had visited. The plants had been cleared through the normal quarantine procedure but she is soon afflicted with severe headaches and loss of memory, eventually falling into a coma. Memory loss begins to afflict other members of the crew but they soon become distrustful of one another, making conclusions based on false assumptions and a misreading of the situation. Fortunately, there are two members of the team who seem to be immune.

Slowly, everyone on Atlantis comes to terms with the sudden re-appearance of Dr. Carson Beckett who, he claims, has been Michael's prisoner for some two years. He tells them what he was doing in Michael's laboratory and they set off to find the kidnapped Teyla. Dr. Keller figures out just how he's come to there at all. Sheppard meanwhile has been using all of his contacts to try and locate Teyla but they come across someone from the past. Teyla in turn has found some of the Athosians alive but they are few in number as a result of Michael's experiments. Michael explains to her just what it is he wants from her as well as his own plans for the future.

Dr. Carson Beckett is removed from stasis because he's the only one who can find the antidote to a mysterious virus that has incapacitated base personnel, beginning with Dr. Jennifer Keller.

Dr. Daniel Jackson comes to Atlantis in search of a lost Atlantian room. The discovery reveals a new race. Daniel and McKay are kidnapped by the new race and are forced to finish a device which may kill the Wraith. Todd feels betrayed and takes the Daedalus.

The new race that stole the device, not to mention Rodney and Daniel, turns out to be an old one meaning very old and well known to the human race. With an ancient grudge to settle, they force Rodney to activate the device against the Wraith with a side effect that could kill millions throughout the galaxy. That makes the Wraith mad so they abscond with the Daedalus to retaliate. Once again we lowly humans have to take control of a messy situation to keep the master races from wreaking havoc in the galaxy.

An alien mind takes up residence in Jennifer's body and seems as confused as her colleagues are by her erratic behavior. Which begs the question; where is Jennifer?