The Welsh mum dubbed the “Merthyr Mermaid” as she swims into the record books in Antarctica; the staff taking a Scottish harbour into a new era; the County Down family dealing the height-restricting condition achondroplasia, also known as dwarfism; and the single dad who is England’s most accomplished high wire artist are just some of the people featured in a new series of BBC One documentary series Our Lives.

The programme returns for a fourth series this summer and across 12 episodes once again tells the uplifting stories of people from every corner of the UK.

It includes Cath Pendleton from Merthyr Tydfil who, at the age of 48, set herself a challenge - to swim an “ice mile” (a swim in waters of 5 degrees Celsius or less) in Antarctica, and ended up becoming the first person to swim a mile inside the Antarctic Polar Circle. Cath’s gruelling training programme sees her swimming in local ponds and bizarrely lying in a freezer in her garden shed. But there’s nothing funny about this challenge - the freezing temperatures could kill her and packs of orcas hunt in the frozen waters.

Our Lives also goes behind the scenes at Aberdeen Harbour. Established in 1136, it is still welcoming ships many these days from around the world, and staff are now aiming to make it one of Scotland’s major cruise ship destinations.

Our Lives will introduce viewers to the Mailey family from County Down. Primary school teacher Tricia is mum to Cara, 12, Noah, five, and one-year-old Bethany. Tricia and her husband Sean’s home is full of love, laughter and the usual family problems with the added complication that Tricia and her two eldest children have the height-restricting condition achondroplasia.

Also featured is Chris Bullzini, a single dad from Somerset who is England’s top high wire artist. Walking the tight rope high above our towns and cities with no safety net or harness, his life and livelihood hang in the balance.

The 12 new Our Lives episodes will be shown over the summer on BBC One. Each is an eye-opening observational documentary about ordinary people doing extraordinary things.

Also in the series are programmes about a fourth generation sheep farmer from the Peak District, the residents of a Swansea council estate maintaining a tradition for keeping urban horses, and a hill walking group from Glasgow’s Asian community.

Rhodri Talfan Davies, Director of BBC Nations, says: “The BBC is determined to do more to reflect lives right across our country. Our Lives does this perfectly by showcasing surprising stories from around the UK that have the power to move viewers and make them smile.”

Nasfim Haque, BBC Factual commissioning editor, says: “This series of Our Lives shows a deep breadth of stories that reflect the diversity of contemporary life in different parts of the UK.”

The series begins on Tuesday 10 August with The Merthyr Mermaid on BBC One.