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Underworld was shown in January 1978 as part of season 15, which I missed for some reason, probably having mourned the loss of Pertwee, while Baker failed to measure up as the doctor, I must have done my school homework instead. However I did read the novelisation by the great Terry Dicks some years later, which I recall more favourably, because it better described the characters, their ship, and the underworld scenery compared to my rather critical view now, seeing it on VHS, even allowing for the passage of over 20 years.
During this season the budgets seemed to allow for some important well-made-in-all-departments adventures, that are long remembered or so well media scrutinised one can't help but acknowledge their importance in the Who universe. Unfortunately, this isn't one of them. Among the cast leaders was James Maxwell as Commander Jackson, Alan Lake as Herrick, pronounced 'Eric', which I found rather funny and introducing Imogen Bickford-Smith (who was hotly tipped to replace Louise Jameson as companion according to the detailed box trivia) in the role of Tala. The Minyon crew were on a quest to find there lost ark type ship the P7E that carried the last of their race and they kept repeating the same line of "the quest is the quest" every other minute, which was a big comedy moment, especially when the Doctor joined in.
The Minyons had gotten the power of regeneration from the Time Lords a millennia ago as medical aid and just kept on using it thousands of time over, whilst on the quest, but this profound fact was badly portrayed as they would surely have given up or gone mad or both, instead of carrying on with salutes and the regeneration process - that had to be induced and was not natural, as with the Time Lords. Herrick's character had only been spoiling for a ray gun fight for thousands of years, possibly the only way to show a hero commit suicide in a show aimed at children. Jackson was a poor grey haired captain figure with no screen presence to fence with the Doctor, and Tala was just blonde icing there only to be re-generated from dying old dear into silver suited space babe.
In conclusion, this was a very mediocre story from a rather dull period of Dr Who but, fortunately, things got better shortly thereafter.