Mathematician Dr Hannah Fry uses Dr Xand Van Tulleken as her guinea pig to test whether the algorithms that dating sites use to match people actually work. While Hannah builds a dating site, Xand meets the scientists investigating online dating - and learns what pictures to use and what to write in his profile. He tries out a 'bot' that has automated a swiping app and has an MRI scan to find out whether his brain is equipped for love. 50 members of the public take part in some mini experiments at a date night - and Xand goes on various dates to test whether the algorithm is better than him choosing randomly.

Horizon investigates a new era of Alzheimer's research, which is bringing hope to millions across the world. New scanning and gene technology is allowing scientists to identify the disease at its earliest stages, often 15 years before symptoms appear. A series of new drugs trials in Colombia, the USA and Europe are showing startling success in reducing beta amyloid, the protein which is a hallmark of the disease. It is becoming clear that changes in lifestyle can prevent the development of the disease. A new system inside the brain has been discovered which clears amyloid when we are in deep sleep, but allows it to accumulate if we don't sleep well. The programme reveals that for sufferers in the early stages of the disease, brain connections can be strengthened and even replaced by absorbing enough of the right nutrients. In an ageing world, where the biggest risk of developing Alzheimer's is old age, these scientific breakthroughs are bringing hope where once there was despair.