S13E21-26
Two alien seed pods are found buried in the Antarctic permafrost and the Doctor realises that they are from a Krynoid, a form of plant life that infects and transforms all animal life on planets upon which it becomes established.
One of the pods infects a scientist at an Antarctic base but the developing Krynoid is destroyed by a bomb set by two men, Scorby and Keeler, who have made off with the other pod for their boss, eccentric plant collector Harrison Chase.
At his mansion in England Chase arranges for the pod to be opened under controlled conditions while a human host - Sarah - is held nearby. The Doctor rescues Sarah, but Keeler is infected. Keeler's transformation into a Krynoid is accelerated by Chase, who has him fed with raw meat.
The creature escapes and goes on the rampage, rapidly growing to giant proportions. Chase decides to turn the Doctor into compost by feeding him into a pulverising machine. The Time Lord escapes, but Chase falls into the machine and is killed. UNIT have meanwhile been called in, and they arrange for the Krynoid to be bombed before it can spread its pods across the Earth.

The season introduces William Hartnell as the First Doctor, an alien who travels through time and space in his TARDIS, which appears to be a British police box on the outside. Carole Ann Ford is also introduced as the Doctor's granddaughter Susan Foreman, who acts as his companion alongside her schoolteachers Ian Chesterton and Barbara Wright, portrayed by William Russell and Jacqueline Hill, respectively. Throughout the season, the Doctor and his companions travel throughout history and into the future. Historical stories were intended to educate viewers about significant events in history, such as the Aztec civilisation and the French Revolution; futuristic episodes took a more subtle approach to educating viewers, such as the theme of pacifism with the Daleks.