Iolo starts his journey during spring in Nant Ffrancon in the Ogwen Valley where he finds one of Britain's most-threatened birds - the twite. Mountain guide Hannah Hughes takes him high up above the stunning glacial landscape of Cwm Idwal for a spectacular view of the high peaks. On the slopes of Snowdon, bluebells bloom, while shepherd Hefin Hughes keeps sheep off arctic plants. On the foothills of the Rhinog Mountains, cuckoos call. Iolo heads to a wonderful hidden river gorge in full flow and listens to the best dawn chorus in Snowdonia in an ancient woodland. He joins Josie Bridges, who is part of a team tracking pine martens that were released in mid-Wales during 2015 after apparently disappearing from the whole of Wales over 50 years ago. A family of pine martens has made its way to Snowdonia, a journey of around 50 miles from the release site. Iolo also discovers a relic of a past landscape. Bird Rock, an imposing crag in the Dysynni Valley, may have once overlooked the sea, and to this day cormorants still nest on the rock - it being Wales's only inland nesting colony.

It's summer and peak season on the summit of Snowdon. It's time for Iolo to get away from the crowds to look for peaceful stunning scenery and spectacular wildlife. In secret hidden sites, he finds buzzards nesting, beautiful dragonflies and night-flying moths being hunted by migrant nightjars from Africa. There's a colony of rare silver-studded blue butterflies who were once living by the sea, but today have been isolated inland as the coastline changed. Iolo also climbs Cadair Idris, the highest peak in the south part of the park and explores extraordinary sand dunes full of colourful orchids along Snowdonia's 30 miles of stunning coast. But summer isn't complete for Iolo without a day on the moors watching the most threatened bird of prey in Britain - the hen harrier.

In this episode, it's autumn and Iolo takes us to hidden wildernesses with outstanding views of the season's colours and the big mountains of Snowdonia. On the lower slopes of the Carneddau range, red squirrels have returned to the national park after disappearing some 40 years ago. Below the magnificent Aber Falls, with its 37-metre drop from the high peaks, jays collect acorns in an ancient woodland. Iolo finds fascinating old sheepfolds and Wales's native black cattle breed on the high slopes - both relics of a time when many more people lived and worked on the uplands and evidence of Snowdonia's changing nature. However, some things remain the same; wild goats rut on the Rhinog Mountains, a tawny owl and bats take up residence in an old church near Dolgellau and, in a nearby wood, dormice fatten up for hibernation. Also, the first whooper swans arrive back from Iceland to a lake below Snowdon - a sure sign that winter is on its way.

In this final episode, it's winter and Iolo endures life-threatening wind chill, ice and snow on the Carneddau high peaks. A cold snap takes him to one of his old roaming grounds on the Berwyn moors where he finds a solitary robin surviving in thick snow. Opposite Portmeirion, the stunning Dwyryd estuary is teeming with winter birds feeding. Crossbills busily eat cone seeds in a forest near Betws-y-Coed. Iolo also heads to Ceunant Llennyrch, a wonderful gorge hidden by an ancient woodland that's existed since trees first colonised Snowdonia after the last ice age. On the foothills of Cadair Idris, he learns that old folk had names for every piece of land.