therein
adverb old-fashioned formal uk/ˌðeəˈrɪn/ us/ˌðerˈɪn/
在那里;在其中;在那点上
in or into a particular place, thing, etc.
It is a thrilling tale of a haunted house and the ghosts therein. 那是一个令人毛骨悚然的故事，讲的是一座凶宅还有里面的幽灵。
Idiom
therein liesidiom 那一点就是，此即
in what has just been mentioned we see the reason or explanation for (a particular situation, quality, or problem)
Her book is simply a collection of memories, told without conceit or self-absorption - and therein lies its power. 她的书只是一本回忆集，既没有自高自大也没有顾影自怜——而这正是该书的魅力所在。