The first thing that grabbed me about the first episode of the story arc collectively known as The Sensorites was the bonhomie between the TARDIS team in the wake of the events of the Aztecs. It's openly acknowledged
IAN: There's one thing about it, Doctor. We're certainly different from when we started out with you.The Doctor puts it well, noting that what started as a of mild curiosity in a junkyard, but now it's developed into a great sense of adventure. Barbara and Ian are happy. They like the Doctor and Susan, they like traveling--though ultimately they want to go home, they seem content to go the long way around.
SUSAN: That's funny. Grandfather and I were talking about that just before you came in. How you've both changed.
BARBARA: Well we've all changed.
SUSAN: Have I?
DOCTOR: Yes, it all started out as a mild curiosity in a junkyard, and now it's turned out to be quite a, quite a great spirit of adventure, don't you think?
IAN: Yes. We've had some pretty rough times and even that doesn't stop us. It's a wonderful thing, this ship of yours, Doctor
The TARDIS has landed within a space ship from the 28th Century, complete with two dead astronauts, Captain Maitland and Carol Richmond. Except they're not dead; they're asleep, placed in an extended slumber by the unknown aliens, the Sensorites, who refuse to allow humanity to leave "this area of space." The awakened team of 28th Century astronauts are trying to break free from the Senorites, but their hope is low; they persuade the TARDIS team to leave after dropping hints about Earth's future, such as the whole southern half of England is now known as Central City; it hasn't been known as "London" since the 24th Century. The Doctor and Ian's help Carol to avoid a Sensorite trap, pulling their ship into whatever heavenly body it's nearest, but the crew persuade the Doctor and Ian that they cannot safely be helped--until they all smell something burning, and discover the TARDIS's lock cylinder has been removed, trapping them outside of it.
As they try to discern next moves, Susan and Barbara go for water, only to be trapped in a different section of the ship, where they are stalked by the third member of the crew, John. He is vaguely zombie-like, slightly menacing. He tracks Barbara and Susan, only to fall at their feet, crying. Touched, Barbara kneels down by him, and holds him, getting from him that he's ill.
Meanwhile, the Doctor is panicking about Susan's well-being (and Barbara's too), and stirring Maitland to force the door, especially after Carol reveals that John is trapped on the other side, the most thoroughly broken member of the crew, and most under the Sensorite control. The Sensorites draw near, with strange high-pitched noises. The Doctor and Ian attempt evasive action, with some success.
John is awoken, giving rise to a brief fear that he will attack Susan and Barbara, but he avows that he will protect Barbara and Susan. "Yes," Barbara murmurs kindly to this squirrelly, rather broken, man, "You protect us."
Back in the control room, a Sensorite--rather a gremlinish little thing--latches onto the window of the ship.
The Sensorites gets a rough ride from Neil Perryman, so I was expecting it to be deathly dull. Halfway through, it isn't. There are two dramatic movements here that kept me interested: First, the Doctor and Susan being in conflict, with Susan (to my mind) being in the right. She wants to talk with the Sensorites (as does he), but she understands their fear much better than he does, and his brusqueness is harmful to establishing diplomatic relations. Through these three episodes, the Doctor calls on Susan to be obedient, and ultimately prevails, but her gentler approach is in fact more productive.
Susan's telepathy is revealed here, and she is altogether more impressive than in past episodes. Carole Ann Ford is quite effective, much less flustered than she is normally required to be, and Susan's brief rebellion is well played.
Barbara likewise stops Ian from attacking the Sensorites on the spaceship, and this, plus the Doctor's not pressing their advantage when he has blinded them, leads the Sensorites ' First Elder to try a negotiated solution. Susan's growing up, and her greater wisdom here, is a great character beat. The Doctor being wrong (though not entirely) likewise works well. When the travelers are brought down to the Sensphere, the Doctor starts adjusting to the atmosphere, and following the path Susan had initially laid out.
The second major plot line is that the Sensorites are actually quite timid, reluctant to hurt the humans, but desperate to save their imperiled world from further infection. In a gambit echoed in The Zygon Inversion, fear leads to bad decisions--in the Sensorites, it's the terrified junior ministers of the Sensephere who decide to kill the humans (thus endangering the cure to the plague that afflicts them).
The travelers and the crew of the Earth ship go down to the planet, and try to negotiate peace. But Ian, who has drunk of the waters received for the lowest caste Sensorites, is struck down by the plague....
The Sensorite faces are well realized; the bodies less so. Raymond Cusack's design is effective, both on the spaceship and on the Sensphere--two very different future aesthetics, each distinguishable from the other, and from the TARDIS.
It's dated, this parable about making peace, but not as badly as all that--it feels rather like an episode of The Twilight Zone, and that's no bad thing.