Here are some spoilers from Digital Spy for the Poison sky however 2 of the below sentances are false:
Donna's mother Sylvia wields an axe.
A jellyfish-like Rutan army makes an appearance (via a Sontaran flashback).
The Sontarans steal the TARDIS.
UNIT prepare to launch a nuclear strike.
Donna kills a Sontaran via the probic vent and booms out "back of the neck!"
The Valiant is put to good use by Colonel Mace.
The current whereabouts (and official title) of Alastair Lethbridge-Stewart are revealed.
Donna punches the real Martha in the face - believing her to be the clone.
Luke has devised a mating programme for his genius recruits.
The Doctor puts on a gas mask to combat the poisonous sky and squeals "are you my mummy?"