Five children on holiday in the Gwynant valley, North Wales, explore a cave on the hill Dinas Emrys and find themselves back in the year 1511. They meet Maroth the wizard and Merric, his companion from a far-off world who is stranded on Earth.

Maroth cannot help them, but he takes them to Elgrin, the leader of the Wizard’s Council. The children have to face many dangers on their way to Elgrin, but they eventually manage to meet him. Elgrin tells them of the key to the Gateway of Time. The children then search for the key and find it, only to have it stolen from them by the evil magician Drascobal. They follow Drascobal and the key is somehow retrieved, but cannot now be used. A twist in the tale helps bring about a thrilling conclusion to the adventure.