26-year-old Hayley’s life has come to a standstill. In the last few years, she has gained 20 kilos and gone from a size 4-6 to a size 18-20, without changing her diet. She also struggles with muscle weakness and constant fatigue, and her hair has started falling out.

After examining her medical history, the team has numerous lines of enquiry to pursue. Renowned haematologist Professor John Gribben wants to investigate what is causing her enlarged spleen, consultant endocrinologist Dr Barbara McGowan wants to know if her weight gain is driven by a hormonal imbalance, and consultant dermatologist Dr Sharon Wong explores possible causes of her hair loss.

While the experts begin their investigations into Hayley’s complex case, Michael Mosley introduces them to another young woman who is struggling with frightening symptoms. In 2017, while on a camping holiday with her mum in the Lake District, Lotte developed a severe headache and it has never stopped. The pain is often accompanied by nausea and sensitivity to light and sound, and it has completely changed her life. Unable to function properly, she was forced to drop out of school before taking her final exams and has been practically housebound ever since.

After a year of these paralysing headaches, things got even worse. One morning Lotte woke up unable to see anything out of her left eye. No one has been able to explain this loss of sight, and Lotte’s mum Lydia is at her wits' end.

For the medical team, Lotte’s symptoms are confusing. Her medical records show that she has undergone a raft of tests, including MRI scans, CT scans and a review by an ophthalmologist, but none of them have shown up any abnormalities. After a lively discussion, two of the experts agree to lead the investigations into Lotte’s symptoms. Show less