Jo first appears in the 1971 serial Terror of the Autons, having been assigned to the Doctor (Jon Pertwee) as a replacement for Liz Shaw (Caroline John). Apparently, she gained the assignment to UNIT because her uncle, a high ranking civil servant, had "pulled some strings". Brigadier Lethbridge-Stewart (Nicholas Courtney) assigns her to the Doctor, who is initially dismayed when he finds out that she is not a scientist, but accepts her because he does not have the heart to tell her otherwise. An enthusiastic, bubbly and sometimes scatter-brained blonde, Jo soon endears herself to the other members of UNIT, especially Captain Mike Yates (Richard Franklin) and Sergeant Benton (John Levene). The Third Doctor is also particularly attached to her, and she is devoted to him, refusing to leave his side even where mortal danger is involved.
There is plenty of danger to go around as well, especially after the Time Lords restore the Third Doctor's ability to travel through time and space. Jo faces the hazards and wonders of travel with the Doctor with courage and plucky determination. Together with the Doctor and UNIT, she encounters such perils as killer daffodils, time-eating monsters, and renegade Time Lord the Master (Roger Delgado). She is miniaturised, hypnotised, flung through time, nearly aged to death, and menaced by giant maggots and ancient dæmons. Over time, Jo also grows more confident and mature, until she is independent enough to stand up to the Doctor, which she does in her last serial, The Green Death in May–June 1973. During the events of that story, Jo falls in love with Professor Clifford Jones (Stewart Bevan), a young, Nobel Prize-winning scientist leading an environmentalist group. At the end, she agrees to marry Jones and go with him to the Amazon to study its vegetation, the news of which the Doctor greets with a mixture of pride and sadness.
