The series begins by following Atlantic grey seals through the year, from courtship to pregnancy and birth. The film also witnesses a pair of guillemots bringing up their single youngster on a narrow rock shelf and takes viewers into the world of bottle-nose dolphins of Cardigan Bay.

Presented by Dame Sian Phillips, this episode looks at some of the biggest challenges and issues facing the marine environment in Wales and the rest of the world today.

As the water starts to warm in the summer, marine giants arrive at the Celtic deep. Some have travelled for thousands of miles to feast on plankton and abundant shoals of fish.

The strange environment of the shallow seas is one that sits remarkably close to shore. In the giant kelp forest, we join a female catshark as she lays an egg deep in the forest floor.