I would have been completely absorbed in this story when I was younger, around ten years old, but now I don’t remember ever seeing it. It seems like they repurposed the scenery from the much stronger "Robots of Death" (1977) for this rather flimsy adventure featuring Tom Baker and Louise Jameson. The “TARDIS” turns up seemingly by accident on the very edge of explored space, just as the “Minyon” race is dealing with a major problem. A spaceship led by “Jackson” (James Maxwell) is trying to locate an earlier mission that carried their entire genetic record in two gold containers. Things get off to a rough beginning, but eventually these brave explorers arrive on a planet inhabited by the "Seers," who work for the "Oracle"—though it’s possible they are actually the “Minyon” people everyone is looking for. One thing this series usually avoided was special effects. They were aware that their budgets wouldn't allow for quality visuals, so they prioritized strong narratives and well-developed characters instead. This one deviates from that approach, and while I did appreciate seeing the shields with ray guns actually shooting lasers, it means a lot of the action takes place against a green screen, which hasn’t aged well. As the title implies, much of the story is set underground—but unlike the natural caves and quarries Jon Pertwee frequently used, these are clearly built in a studio, and they sometimes look pretty silly. The last portion feels hurried, and overall this gave me the impression that the BBC was short on money, leading to this drama as an attempt to save costs. It somehow suggests that Baker's run as the "Time Lord" is nearing its end, and it’s one of his least memorable stories.

Source on TMDB