It's been five years since Hiccup and Toothless successfully united dragons and vikings. While Astrid, Snotlout and the rest of the gang are challenging each other to dragon races, the now inseparable pair journey through the skies, charting unmapped territories and exploring new worlds. When one of their adventures leads to the discovery of a secret ice cave that is home to hundreds of new wild dragons and the mysterious Dragon Rider, the two friends find themselves at the center of a battle to protect the peace.