In this moving film Horizon follows the Loughran family in their fight to save the life of their daughter Sheila who suffers from cystic fibrosis. They lost their youngest daughter Ann to the disease in 1974 at the age of 15, and now as the health of their third daughter Sheila deteriorates, they must face the prospect of losing a second child. The current shortage of donor organs means that Sheila's only hope of survival is a rare and controversial operation that requires her two surviving siblings to undergo an arduous and potentially fatal operation.

An X-ray of Shelia's lungs Cystic fibrosis (CF) is the most common genetic disease in this country and it is incurable. The lungs of people with cystic fibrosis become covered with a sticky mucus making them extremely susceptible to bacterial infection. Over time these infections badly scar the lungs, until eventually they stop functioning. The defective CF gene is harmless when only a single copy of the gene is inherited. However, both the Loughran parents carry the gene, giving any child they may have a 25% chance of being born with cystic fibrosis. In fact two of their four children were born with the condition.

Horizon joins the family at a time when Sheila's health has deteriorated to such an extent that she requires oxygen 24 hours a day and has only months to live. Although on the waiting list for a donor lung, with 50% of patients dying while waiting to receive a transplant, Sheila's chances are not good.

The family has become aware of a controversial new operation, pioneered in the UK by Professor Sir Magdi Yacoub at Harefield Hospital. The technique, known as Living Donor Lung Transplantation, would involve removing Sheila's diseased lungs and, in an extraordinary three-way operation, replacing them with a lobe from one of the lungs of each her two siblings.

There have been six of these groundbreaking operations carried out in this country. However, only three patients have lived longer than a month. There is a clear moral dilemma - with such a low success rate, is it ethical to put the lives of two healthy people at risk? Even if the operation is initially successful it may only give Sheila five more years to live, by which time her new lungs are likely to fail again.

Damian Loughran Sheila's brother and sister, Damian and Josephine, feel compelled to do anything they can to save their dying sister. They undergo stringent tests before being certain that they are compatible donors and fit for surgery. They will have to face the risk of haemorrhaging and infection, both of which could potentially be fatal. After the operation both donors will be left with a 20-25% permanent loss of lung function. Despite these dangers, Damian and Josephine remain determined to proceed.

As all three of their children are wheeled in for the 12-hour operation, Mary and Harry Loughran's emotion is apparent. A day later, Sheila is breathing with her new lungs, but it is not long before complications arise. She is unable to absorb food and develops an abscess on her lung. Sheila is kept under sedation and so is unaware of these complications. Sadly, three weeks after the operation, Sheila loses her fight for life.