eucatastrophe
A catastrophe (dramatic event leading to plot resolution) that results in the protagonist's well-being.
But at the story of the little boy (which is a fully attested fact of course) with its apparent sad ending and then its sudden unhoped-for happy ending, I was deeply moved and had that peculiar emotion we all have – though not often. […] For it I coined the word ‘eucatastrophe’: the sudden happy turn in a story which pierces you with a joy that brings tears (which I argued it is the highest function of fairy-stories to produce).
Where it comes from
From eu- + catastrophe, coined by English author and academic J. R. R. Tolkien in 1944 as part of a letter: see quotation.
Adapted from the Wiktionary entry eucatastrophe (CC BY-SA 4.0).