On a rare mandatory day off in Atlantis, a handsome and extremely persistent scientist named Mike Branton actually talks Elizabeth Weir into relaxing and spending her lunch hour with him. Weir reluctantly agrees and, to her surprise, has a good time.
Other team members are similarly remembering how nice a day off can be. Teyla teaches her friend Dr. Hewston about Athosian Bantos fighting. Sheppard has less luck impressing Ronon with the majesty of golf, and when Ronon's reciprocal attempt to teach Sheppard a Satedan game leaves the colonel battered, the two men call off their recreational-sports exchange program and settle for snacks, drinks and laconic male bonding. For his part, McKay ditches a planned fishing trip with Carson Beckett so he can snuggle up with his on-again off-again date, Katie Brown, in the botany lab.
Beckett had been looking forward to the fishing trip with McKay for days, and now he can't find anyone to take McKay's place. All his other friends have plans and turn him down. Frustrated, Beckett returns to the infirmary, where the on-call physician is suffering from a migraine. Beckett compassionately sends her home to her quarters and takes over her shift himself.
That's when an explosion rips through one of the city's main corridors. Beckett arrives to find three people dead and many seriously injured, including Teyla. As he rushes her into surgery, McKay and Zelenka race to figure out what happened. Disturbingly, they discover that the bomb wasn't an object, but a person: Teyla's friend Dr. Hewston.
McKay recalls that a few days earlier Hewston and another scientist, Dr. Watson, accidentally turned on an unknown Ancient device. Though the incident seemed harmless at the time, a quick investigation now reveals the awful truth: The device is another failed Ancient weapon. It creates living bombs by pouring specialized radiation into the body of anyone nearby, which causes the body to produce an explosive tumor. They have just seen the results of this terrible process in Hewston. Now they need to find Watson - fast.
Watson was also injured in the blast and is now in surgery. The team clears the area around the infirmary, but even though Watson may explode any second, Dr. Beckett refuses to leave the man's side. Instead, defying his friends' pleas, Beckett dedicates himself to saving Watson, even if the man turns out to be the last patient he ever treats.