The first episode reveals the dilemmas faced by the London Ambulance Service as they are forced to bump patients down the queue so they can prioritise the sickest, deal with time-wasters and cope with the ever-present threat that a major incident is just a 999 call away. An ordinary day in London means dozens of car crashes, overdoses, suicide attempts and - statistically - 28 cardiac arrests, where every second can make the difference between life and death. By 11.00am, seven cardiac arrests have already come in, and now there are two more people fighting for their lives. As one crew battles through traffic to try to save a dad of two, across the city, another faces a difficult decision - whether to stop resuscitating their patient, knowing full well the impact it will have on the family. The brain of the service is the control room and, when a number of stabbings, suicide attempts and a double shooting flood the 999 phone lines simultaneously, they threaten to overwhelm the service. And when there is an unexpected spike in 999 calls - far outstripping the number of ambulances available - drastic action has to be taken. Some emergency callers have to be told an ambulance cannot come to them so that the service can prioritise reaching those in most urgent need. Multiple calls about an explosion in a flat come in and herald the control centre's worst fear - now, the whole of the London Ambulance Service has to step up.