It's a big day for Rodney McKay who has decided to propose to Katie Brown. Before he can get the words out of his mouth however, alarms sound and he and Katie find themselves trapped in her lab with no way of communicating with anyone. They aren't the only ones trapped however as all of Atlantis seems to have been locked down as part of an automatic quarantine procedure. Sheppard and Teyla are trapped and his biggest worry seems to be that she might go into premature labor; Samantha Carter and Radek Zalenka are stuck in an elevator; and Ronon and Jennifer Keller are trapped in the infirmary. Things go from bad to worse when the system activates it's ultimate defense against an infection - the city's self destruct mechanism. With the computers down, no one knows just how long they have to shut down the city's main power supply, the only way to prevent catastrophe.