Biography
Tegan joined the Doctor on February 28, 1981. While she was on her way to Heathrow to start a new job with Air Australia, one of her cars tires got a puncture. She entered the TARDIS, thinking it was an actual police box. The Master devised a plan to trap the Doctor and in the process of carrying it out killed Tegan's aunt Vanessa with his Tissue Compression Elminator. (DW: Logopolis).
She became very good friends with Nyssa and was distraught after Adric's death and Nyssa's leaving the TARDIS crew.
