Iolo Williams goes on a journey around Wales to meet people who work on the sea, those who have a passion for the sea and he explores the extraordinary wildlife that lives in the sea. In this first episode he travels along the North Wales coast visiting the second largest wind farm in the world near Rhyl and goes lobster and crab fishing near Llandudno. As he reaches the Menai Straits Iolo dives to explore the wreck of the old battleship HMS Conway and finds unusual creatures called sea hares off the south west coast of Anglesey

IoIo Williams continues his journey around Wales, visiting the harbour of Trefor on the north Llyn coast, an area renowned for its sea kayaking. The presenter learns about a 150-year-old shipwreck near Nefyn, where the slate cargo has been transformed into a reef brimming with sea life, sets sail on a fast offshore racing yacht from Pwllheli marina, and discovers the nocturnal underwater world lying just off the coast from Criccieth.

Iolo Williams continues his journey around Wales, travelling from New Quay in Cardigan Bay to an outcrop 15 miles off the Pembrokeshire coast called the Smalls. Once there, he dives with friendly Atlantic grey seals, goes lobster fishing in a traditional, wooden boat, and discovers the underwater remains of a sea defence wall dating back to 1833.

Iolo Williams concludes his journey around Wales, joining a shark-tagging project in the Celtic Deep, investigating a sunken seaplane in the murky waters of Pembroke Dock, and paddle-boarding off the Gower Peninsula. In his final stretch from Swansea to the Severn Bridge, the presenter meets a commercial diver in the city's marina and goes on patrol with the Cardiff police boat team. Last in the series.