The migration of the painted lady has long fascinated scientists, artists and nature lovers alike. The longest butterfly migration on earth, it sees millions of these delicate creatures travel from the desert fringes of north Africa, across thousands of miles of land and sea, before settling in the UK. However, the migration has never truly been understood, the mysteries of the painted lady never unravelled - until now. This documentary reveals the secrets of this extraordinary phenomenon. Observed, investigated and analysed by presenter Martha Kearney and entomologist Dr James Logan, it employs groundbreaking techniques to unlock the secrets of the painted ladies. At a time when more than a third of Britain's butterfly species are classed as under threat of extinction or have already vanished, it documents the largest butterfly migration into the UK.