Spoiler warning: Spoilers for the complete film.
The killer is Quinn’s future self, and Lucy defeats him before choosing to remain in 2003 with Summer. Time Cut gives the sisters a life together at the cost of the original future in which Lucy grew up.
Why are there two versions of Quinn?
The younger Quinn befriends Lucy after she travels into the past. His older self has returned from a different future, carrying resentment over the bullying and humiliation he experienced.
Lucy intervenes in the incident that helped shape that grievance. Her friendship with the teenager should not be confused with approval of the adult killer’s actions, which remain his responsibility.
The two versions let the film distinguish a person facing an injury from one who has organized a later life around punishing others for it.
Does Summer survive?
Yes. Lucy kills the older Quinn during the final confrontation, and Summer is freed from the death that defined the family Lucy knew. Summer can also embrace her relationship with Emmy.
Saving her does not preserve every other event unchanged. Lucy’s original timeline depended on a family shaped by Summer’s death; after the rescue, her own place in that future is no longer available in the same way.
The film chooses a branching emotional outcome over a fully consistent explanation of every time-travel paradox. It shows where Lucy decides to live without supplying a scientific model that resolves every contradiction.
Why stay in the past?
In 2003, Lucy has formed direct relationships with the sister she previously knew only through loss and the young Quinn whose future can now be different. Staying gives her a place in that new arrangement.
Her choice does not mean the life she came from was imaginary. It means changing the event that governed it has consequences she cannot simply reverse while keeping Summer alive.
The happy ending is therefore specific: the sisters can share a present. It is not a guarantee that time travel allows Lucy to retain every version of the family she might want.
Sources: official ending breakdown.