With the entire city ordered to take a personal day the team does their best to unwind and enjoy their leisure time. Without warning a mysterious explosion rips through the city killing and injuring unsuspecting citizens. Dr. Rodney McKay (David Hewlett) and Dr. Carson Beckett (Paul McGillion) arrive on the scene to find Teyla (Rachel Luttrell) critically injured by shrapnel from the explosion. Unable to find explosive residue, the only leads they have to go on are eyewitness accounts saying one of the base-scientists just suddenly exploded. McKay, realizing the scientist in question is one of his own, quickly puts the pieces together to learn that the incident is likely the cause of an Ancient device mistakenly activated days earlier by two of his scientists. The device which is actually a type of weapon infects nearby organisms with a tumor-like explosive that detonate after reaching critical mass. Knowing that the other scientist is still somewhere on the base the race is on to track down the man who is literally a walking time bomb before he explodes and injures more people.