In the end, it is discovered that the Sensorites are being poisoned by humans on the planet, who have become monsters. The offenders are stopped, and sent home to be treated for madness. It's an unexpected twist, and not one that seems particularly brilliant. There isn't really a set of clues that leads to this discovery, but rather, it's tossed in at the last minute. Slightly disappointing.
What "The Sensorites" does well is deliver a sci-fi experience. The look of the alien world is pretty, well, alien. The set builders use lots of curved lines to differentiate the architecture of the Sensorites. Being that this is made in 1964, and predates most filmed science fiction, Doctor Who really had room to play at making a new environment, and they do so very well. There is also a general level of creepiness infused in much of the adventure, owing to the slow burn unfolding of the plot, wonderfully capturing a tone of unease in this strange place.
"The Sensorites" DVD comes with some pretty great extras. Twenty-one minutes are devoted to looking into the writer of this serial, Peter R. Newman, who, strangely enough, has only one other screen credit. There is a seven minute extra examining what a Vision Mixer does, and two minutes cover how the Sensorites' voices were developed by said mixer.
The commentary track has a lot of voices on it, but it is moderated to avoid being too confusing. The performers behind Ian and Susan are present, as well as some guest stars, the director, Frank Cox, and a couple of crew members. In all, there is a wealth of information to be gleaned from these tracks. Also included is a photo gallery and PDF materials, which include some design sketches. Pretty neat.
For a serial so old, "The Sensorites" looks good. The audio and picture have been remastered, of course, and perhaps that's why it took so long to reach release. The work is fantastic, and one may be surprised at how nicely it is presented, considering.
Buy Doctor Who - "The Sensorites," available now on DVD.