Tegan is one of very few companions to have worked with multiple Doctors. The only other ones that have worked with multiple Doctors are Jo Grant, Brigadier Lethbridge-Stewart, Jamie, and Sarah Jane Smith. Tegan met the First, Second and Third Doctors in the Five Doctors episode, and she first started off with the Fourth Doctor before he regenerated into the youthful Fifth Doctor. It doesn't stop here either. When Colin Baker became the 6th Doctor, there was a program called Jim'll Fix It, and there was a specially made episode called A Fix with Sontarans in 1985 and it featured Tegan with the 6th Doctor. Though it should be mentioned, Peri was supposed to be the companion, but Nicola was occupied at the time. So they brought Janet in, who had worked with Colin before in Arc of Infinity, and it just worked brilliantly. The chemistry between them was actually quite good. Not as perfect as with Peter Davison's Doctor, but very nearly.