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.

Episode two joins the staff in the nerve-centre control room on the night shift, when amongst the thousands of calls they receive, they also have to contend with hoaxers, frequent callers and run-away patients. At one point, 125 patients are waiting for assistance and the ambulance allocators are forced to prioritise the sickest patients, even if that means bumping others down the queue. An ambulance crew is on their way to an elderly faller who's been waiting on the floor for three hours, but he's left even longer when they are diverted to an urgent call for a woman who says she is having a miscarriage. When they get there, they discover all is not as it first seemed and the crew is torn between frustration and compassion for a patient who's clearly troubled. Meanwhile, another crew assist a 94-year-old who entertains them with surprisingly candid talk of her love life. As her condition deteriorates, the decision is made to take her into hospital. The crew are all too aware that this may be the last time she leaves home. Across the city, a call handler spends an hour and a half on the phone to a suicidal patient who is running away from the crews trying to track him in the busy streets of London. A dementia sufferer is all too desperate for an ambulance. She has called 15 times tonight, and even after visits by clinicians who agreed she didn't need their medical assistance, she continues to call. If they keep sending ambulances to her, there'll be fewer for other patients in the area, so the service faces a difficult decision to say no to her heart rending pleas for help.

The London Ambulance Service brace themselves for a demanding weekend shift. As Londoners head out to play, the Ambulance Service often has to pick up the pieces. They need prepare for a torrent of cases, all with their own unique challenges. In Brixton, advanced paramedic Rich helps a man who's out of control and lashing out in a nightclub after taking a drug overdose. In east London, a man who's travelled all the way in from Essex to eat at his favourite kebab house has been viciously assaulted. In the control centre, it's fun and games from nine months ago that's causing the phone to ring - five babies are being delivered on the phone tonight. By Saturday night, the streets of London bring a huge challenge for the service - as the pubs begin to close, the violence is escalating. The ambulance dispatchers are forced to make tough calls about who gets an ambulance quickest and who will have wait - when a stabbing comes in they have to divert a crew from a six-year-old who's fallen from a bunk bed. It's not an easy decision for any of them. A couple of hours later, the same crew attend their second stabbing of the night, and the seventh across London this evening. When, at the end of the weekend, a call for a miscarriage comes in, it's the type of emergency that the ambulance staff are trained for, but all the preparation in the world can't always shield them from the emotional toll that sharing a personal tragedy can have.