Well, that was a slightly terrifying episode, and Donna really needs to understand the bit about changing history. Apart from that, she is smart, quick-thinking, independent and (dare I say it) plucky. 
Tonight The Doctor and his plucky assistant, Donna, arrived in 1926 on the day Agatha Christie disappeared. Two mysteries unfold; where did Agatha vanish to, and why is there a giant wasp attacking Lady Eddison's guests?
The plot follows a crime novel storyline, as the Vespiform thinks that human life is lived out in the way Christie describes in her books. At this point, we can be thankful he wasn't watching Midsomer Murders!
Lady Eddison, presumably unwittingly, was assisting the Vespiform - Reverend G in study with the lead piping - by wearing a pendant presented to her forty years ago.
To stop the Vespiform, Donna threw the pendant into the lake and consequently giving Christie amnesia, and thus not messing up established events.
This episode was very intelligently put together, and no sonic screwdrivers brandished. Donna once again showed herself as an able and resourceful assistant - eg, knowing drowning wasps kills them, and the best way to shock The Doctor. Hopefully the next episode isn't a let down by comparison, but we'll have to wait two weeks to see that due to a certain annual European Cheese Fest (with Irish joke act!), next week.