Richard Basehart only appeared briefly in the Voyage episode "The Monster's Webb" reportedly because of an illness back in 1966. There seems to be some controversy as to the real reason for his absence in this episode.
It is apparent that there is a double for many of his scenes. Unfortunately, this hurt the episode because the double's scenes were shot so badly. What could they do other than to re-shoot much of the footage with Basehart. The budget restraints, obviously, would forbid this.
It was reported that Mr. Basehart fell ill with an ulcer that put him in hospital. Whatever the illness it was serious enough to warrant the hiring of a double to finish out his scenes. He also missed the next two episodes only reappearing in the final show of season two.
In addition it was alleged that discussions were being held as to replacing Basehart in the series with actor Gary Merrill. Merrell appeared in the next episode as Admiral Parks.
It was no secret that many on the show, including Basehart, may not have been happy with the direction it was taking. The monster of the week scripts seemed to hurt the show. The long hours, hard work and stress appearing in a weekly show like Voyage can be hard on anyone.
Whatever the reason for his absence, whether an illness or other problem, it was obviously resolved.
Let me make it clear that Richard Basehart was a great actor and human being. I admired him very much and without him there may not have been a Voyage series.
If anyone has anything else they would like to add about this please do.