Un soir en détention, Snape fait réaliser à Hermione la potion Fantasia, ou potion à fantasme. Cela va transformer la relation qu'ils entretenaient jusqu'alors.

Hermione quite literally crashes her way back through time by roughly twenty years. There is no going back; the only way is to go forward. And when one unwittingly interferes with time, what one expects may not be what time finds...