S4, E1: When Rodney McKay (David Hewlett) realizes Atlantis has started shutting down artificial gravity and collapsing protective shields to compensate for a damaged power system he turns off all non-essential systems. But when the problem persists McKay is forced to collapse the shield to the point where it is only protecting Atlantis' main tower leaving the rest of the city exposed to space.With the protective shields down and an asteroid belt fast approaching Lt. Col. Sheppard (Joe Flanigan) leads a group of Atlantis personnel on a desperate jumper mission to clear a path for the city. Meanwhile Dr. Keller's (Jewel Staite) attempts to resuscitate a comatose Elizabeth Weir (Torri Higginson) reveal that her patient has severe brain swelling and will require radical surgery. McKay runs the calculations and realizes there's an even bigger problem: in just 28 hours the city's shields will fail completely and all of Atlantis' inhabitants will succumb the vacuum of space.