Iolo Williams tracks Wales' rarest and most elusive mammal, the pine marten, and sees ospreys fishing in the Cambrian Mountains of mid-Wales.

Iolo Williams looks for cuckoos in the Elan Valley, red squirrels genetically unique to the Cambrian Mountains, and golden plover on the hills surrounding Plynlimon.

Iolo Williams tracks goshawks and finds remnants of prey and chicks in a huge nest. He also looks for rare nesting curlew and a stunning day-flying moth called the Welsh clearwing.

In his final trek, Iolo looks for a bird you wouldn't expect to see in Wales as it is meant to be extinct here. But a golden eagle is secretly living in the Cambrian mountains.