One the best touches that sets "Annihilation" apart tonally from other moody sci fi movies is the recurring use of the song "Helplessly Hoping" by folk trio Crosby, Stills & Nash, which is used as a theme for Lena and Kane. The lyrics refer to a "he" that waits silently for a "she," who never opens up to him, and he can't decide whether to run away from her or not.
Lena and Kane's marriage is shrouded in secrecy and for some, it's one of the more confusing elements of the film. Does Kane know about the affair? Are they doomed to keep drifting apart anyway? Kane and Lena certainly have chemistry and inspire deep happiness in one another at times. But the more they keep from each another, they more "confusion has its cost," as the song goes.
The refrain of "Helplessly Hoping" sums up Kane and Lena's problems with a clean bit of wordplay and can clarify some of the ambiguity around their relationship. They're wildly different individuals ("They are one person"), who are drifting apart ("They are two alone"), but still hoping to maintain the relationship itself, which they've been building for years ("They are three together").
They're both radically transformed by the Shimmer in the end and are reunited, so hopefully they can work it out ("They are for each other"). While you couldn't really say "Annihilation" has one of the best movie soundtracks, it does get the most mileage possible out of its lone pop track.
