Graham Merrill (Bill Travers) passes a pet shop on his daily walks about London and takes an interest in an otter in the window, eventually buying and naming the animal Mij. The otter wreaks havoc in his small apartment and together they leave London for a rustic cottage overlooking the sea on the west coast of Scotland. There they live as beachcombers and make the acquaintance of Dr. Mary (Virginia McKenna) from the nearby village, and her dog Johnny. Mij and Johnny play in the water and bound across the fields together.
Mij's inquisitive and adventurous nature leads him some distance from the cottage to a female otter with whom he spends the day. Ignorant of danger, he is caught in a net and nearly killed. The humans find him and help him recover, later building a swimming pool for him so that he may swim at ease without getting into trouble.
Not long after, Merrill goes to London to look after some affairs and leaves Mary in charge of Mij. While being exercised afield, Mij is killed by a ditchdigger, who didn't realize he was a pet. Merrill returns and is crushed to discover the death of his beloved otter. Some time later, Merrill and Dr. Mary are surprised by a trio of otter youngsters approaching the cottage. He happily realizes they are Mij's children who have come to play in their father's swimming pool.