Tommy Knight was not available for filming of this episode due to his GCSE exams. This story is therefore the first not to feature Luke Smith. As a result of this absence, Elisabeth Sladen becomes the only cast member to have appeared in every episode to this point.
Alchemy was a very real practice that started in Europe around the time of the Renaissance. Its main goal was to transform base metals into gold (transmutation), but it also was seeking an elixir that gave immortality. Alchemy was never successful, but it did form the basis of chemistry and biology, which are true Sciences.