The TARDIS crew arrive at an unnamed planet and witness the death of Abu, who is vaporized, but are able to save Vana (Madeleine Mills). The entire race of the humanoid Gonds is being victimized by the crystalline Krotons. The brightest of the Gonds are chosen to be “companions” of the Krotons and are never seen again, having had their lifeforce absorbed. Selris (James Copland) is leader. His son Thara (Gilbert Wynne) is the only real opposition to the Krotons, who arrived in a spaceship years ago.
Thara leads a rebellion and attacks the Kroton teaching machines in the Hall of Learning. Zoe and the Doctor try the teaching machines and are chosen to be companions. They are summoned to the spaceship, called the Dynotrope, where Krotons are created in chemical vats and which is powered by intellect. Unfortunately, when the ship crashed, not enough Krotons were left to maintain the ship and the survivors were forced to absorb the minds of their Gond victims. Jamie is captured.
Gonds Eelek (Philip Madoc) and Axus (Richard Ireson) turn against the Krotons and opt for war. Selris has less dangerous plans, but Eelek has him arrested and attempts to betray him, the Doctor, and Zoe to the Krotons, if they will leave the planet alone. The Krotons want them, as their “high brains” will be able to fly the Kroton ship. Selris dies in the Dynatrope but gives the Doctor acid to put in the Krotons’ vats. Jamie, having escaped, attacks the ship with more acid. The ship and the Krotons are based on Tellurium, which is highly vulnerable to acid. They are destroyed, the Gonds are freed, and the time-travellers depart.
The four-part serial was considered kind of ordinary Doctor Who fare, but it was one of the few Second Doctor stories not to be erased. Later thought was that the story was well told, some of the minor characters were nicely portrayed, and Zoe was particularly effective as a companion; she passed the Krotons’ math-test with higher marks than the Doctor, just beginning to set a new standard for female companions, and when he pulled his bumbling Columbo act on the Krotons, she picked it up right away and made a good foil.
As for Jamie, nobody even suggested he take an intelligence test, the big 18th Century lummox, but he rushed bravely into the Dynatrope to save them despite almost certain death, not knowing that they had already escaped, which was very sweet. They were becoming a perfect pair of companions for that Doctor; too bad they only had three more episodes to go.
At one point, the Doctor was threatened by a tentacle coming out of the wall, with a device like an eye on the end. It looked just like the one in the 1953 version of War of the Worlds. The Kroton robots were more amusing than frightening, as their heads spun around when they were thinking. Their voices were deep and sonorous, like the disembodied brains called Providers in Star Trek’s Gamesters of Triskelion, but I swear one of them had an Australian accent.