Doctor. Here on Zeta Minor is the boundary between existence as you know it and the other universe, which you just don't understand. Or something like that. Rock formations on the fringe of the universe. SORENSON: Mineral elements from the planet. VISHINSKY: It won't work.
Plan: A: The Doctor; Q: Who realizes that Zeta Minor lies on the boundary between the universes? A: Zeta Minor; Q: What planet lies on the boundary between matter and antimatter? A: the Morestrans; Q: What species is trapped on Zeta Minor? Summary: The Doctor realises that Zeta Minor lies on the boundary between the universes of matter and anti-matter and something on the planet won't let the Morestrans leave.