Original publication date: 1967, Little, Brown
When an ancient scroll and necklace are stolen from the Mycetian League, Chuck, David, and Mr. Bass are called into action to help recover them. This means a trip in the rocket to Wales (Carn Bassyd in the Rhinogs, specifically), and then a drive to Stonehenge and London to recover the stones from the necklace. These have been spread among different owners, and have caused the new owners - a doctor, an architect, an artist, and a fashion designer to begin introducing attributes of Mycetian culture into their work.
David and Chuck also learn that Mr. Bass's Elder Grandfather was the founder of the Mytcetian League during the time of King Arthur. His enemy, a being of pure evil called Narrow Brain, continues to seek the destruction of the Mycetian way of life. Defeating him will require both retrieval and translation of the scroll, as well as a trip to Basidium.
Spanish language edition:
Mr. Bass y el Tiempo, Editorial Acme, Coleccion Robin Hood #197, 1974