Brac was a young Voddod warrior whose ship sustained a hit from the enemy. As his ship headed for Earth, he ejected, and the ship crashed into the building where a UNIT Christmas party was being held.
The Fourth Doctor, the Brigadier and several UNIT troops confronted Brac in the woods nearby. The Doctor made a deal with Brac: if he would disable his ship so UNIT could rescue Harry Sullivan and Sarah Jane Smith from the building rubble, the Doctor would take him back home in his TARDIS. (PROSE: UNIT Christmas Parties: Ships That Pass)