The islanders are dismayed when the badly-behaved Big Boy Cousins come to stay, but Katie Morag thinks they are wonderful. She joins in with them as they cause havoc in the village and enjoys disobeying Grannie Island. But when Grannie Island gets angry, Katie Morag realises that really her Grannie is sad inside, and she feels sad and sorry too. So Katie Morag become the leader and makes the Big Boy Cousins join her in making amends.