The idea is that the ship blows up because they're dead (also it's a clumsy echo of Earthshock: Adric stays and dies, Turlough runs and survives).It was a great story and I absolutely loved seeing the Brig again and the dual timezone shenanigans were great. Only thing that bothered me was that the undeads' dilemma was not terribly plausible. If the goal was just to die, certainly they could have just blown up the ship (like happened in the end) or flown it into a sun or something. There is no way they could have regenerated through something like that.