About Doctor Who Season 0, Episode 42 :

The Sixth Doctor (Colin Baker) is working the controls in the TARDIS, and he accidentally teleports his former companion Tegan Jovanka (Janet Fielding) on board. She is less than happy about this, and reluctantly agrees to help the Time Lord. He reveals that two Sontarans are on board, and they possess a powerful vitrox bomb with which they intend to blow up the time machine.