This was not a complete disaster, which given the high standards set in the previous episode is something to be proud of. But personally I didn't think it was quite as good as that episode, even if it did hit some high notes.
So the Daleks plan is revealed... to destroy every universe in existence, leaving just the Daleks. Every new series companion is brought together in one place as the Daleks plan near fruition but thanks to the Doctor's failed regeneration his hand has regenerated into a more human version of him; and left Donna a bit more of a Time Lord at the same time. Using her new found knowledge she stops the Daleks who are then destroyed by Human-Doctor in a fit of rage, at the same time she sends all the planets bar home to where they belong. Davros perishes moments after revealing to the Doctor that he may not use weapons, but he uses people he meets as weapons themselves.
The Doctor and his companions fly the Tardis together and bring the Earth home and he drops them back where they belong: Sarah Jane says goodbye and heads home to Luke, Martha and Mickey head off with Captain Jack to Torchwood, Rose and her mother are stranded back in the alternate dimension with the Human-Doctor, and the Doctor and Donna are finally left alone.
Donna and the Doctor soon realise she cannot continue to exist as she is; her Time Lord brain was not designed for a human body and she is dying. The Doctor and Donna realise there is only one thing to do: erase Donna's memories of the Doctor so she doesn't attempt to use her Time Lord abilities. Donna protests but the Doctor does it... leaving Donna with her family... back to being just a temp from Chiswick.
It was a good episode, but... Donna's fate was the most depressing ending to a series so far. If she had died it would be bad enough, but instead she lives out an existence she herself had said was not how she wished to live. Her own personal Hell. :(
Kudos to Catherine Tate. Despite my love of Martha, I have to say Donna was the best companion of the new series. I shall miss her.