The Doctor, portrayed by Tom Baker, is occupied with the TARDIS when another Romana appears—though something feels amiss. The question arises: could she have regenerated? It seems that she has, and Lalla Ward takes over the role. Fortunately, the story unfolds against the backdrop of an old quarry as they unexpectedly find themselves in the midst of a conflict between the Movellans and their long-standing adversary, the Daleks. Tim Barlow plays Tyssan, representing the local humans being exploited by the Daleks to dig deep underground—but for what purpose? It isn’t long before our time travelers realize they've returned to Skaro, and that those digging are searching for something or someone who could potentially change the balance of power between the warring factions. The Movellans are also aware of this plan and enlist the pair’s help. Naturally, the Doctor knows he shouldn’t choose sides, but maintaining neutrality proves difficult when neither group seems particularly ethical. Terry Nation was among the stronger writers for the series, and combined with a good performance from Baker and plenty of action sequences, this makes for an enjoyable drama that relies on a well-constructed plot and familiar villains to introduce the new character. Ward wasn’t my preferred companion; there's just something a little too polished about her contributions, but she begins acceptably as explosions erupt and a large-scale battle approaches. It isn't exceptional, but it's not disappointing either.

Source on TMDB