It's spring and the nesting season is in full swing. While hundreds of dotterel are taking a rest on the Black Mountains during their journey from Africa to their breeding sites in Scotland, peregrines are already nesting in an old quarry in the central Beacons. Next to the largest natural lake in south Wales, water voles are managing their ditches. Iolo Williams explores the most crooked church in Britain and an old gunpowder works and finds that a 300-year-old stone wall reveals the history of this magnificent landscape.