Oh no, not another stowaway! Tegan is the fifth companion, following Steven, Zoe, Sarah Jane, and Adric, to start her Dr Who career by stowing away on the TARDIS. Those TARDIS people just have to remember to keep the door closed! In Tegan's case she wasn't really a stowaway, she had been driving to Heathrow Airport when her car had a flat tire, so she decided to go into a convenient Police Call Box to call for some roadside assistance. Oops, you wouldn't think you'd have to worry about getting lost in a police call box, but that's what happens to her.
She witnesses the Doctor's regeneration and continues travelling with the Doctor and his other companions. During all her travels she was trying to get back to Heathrow airport in 1984, and her job as a stewardess. The Doctor kept promising to take her there, but, you know, things came up, so when she ended up in 1984 as an incidental result of a Dalek invasion she decides to jump ship.
Janet Fielding (born Janet Mahoney, January 1957, Brisbane, Australia).