Warning: Blindfold yourself if you don't want spoilers from Bird Box the film and the book.
Bird Box bookended its story by closing with the return of a supporting character from the beginning. When the mysterious creature attacks first started happening, Sandra Bullock's very pregnant Malorie was just leaving the doctor's office with her sister Jessica (Sarah Paulson). The doctor treating Malorie was Dr. Lapham, played by Parminder Nagra (E.R., The Blacklist, S.H.I.E.L.D.). At the end of the film -- five years later -- Malorie ran into Dr. Lapham again at the school for the blind that served as a sanctuary for survivors.
Bird Box director Susanne Bier explained why Dr. Lapham made that surprise return to the story:
I felt that it needed some real hope. And I thought having someone else survive, and having someone who she met at a point where she was in a kind of rejecting life, and having to meet at that point [where she's embracing it] --- I felt was a really warm and hopeful element to the ending. And it also makes us think, 'Oh, there are other survivors!' I think it does infuse the ending with a kind of grander hope than if she had not been there.
It also kind of makes me wish Malorie had just stuck with Dr. Lapham. They were together on the same day this all started, and who knows how much Malorie's life would've been different if she'd followed Dr. L's path. Then again, we don't know when the doc got to the sanctuary, or how tough her own journey was. And if Malorie took that shortcut, 1) the movie wouldn't be long or dramatic enough, 2) she would've never had Girl in her life, and 3) she never would've gotten to meet Tom (Trevante Rhodes) or named Boy after him.
And, as Susanne Bier told People, it does bring Malorie's story full circle to go from Dr. Lapham seeing Malorie has a pregnant cynic to Dr. Lapham seeing Malorie has a hopeful mother.
The ending does partly match the ending of Josh Malerman's 2014 Bird Box novel. Malorie and the kids end up at Rick's sanctuary. He's said to be one of 108 people in a refuge with no windows. The film is slightly different, and adds the Dr. Lapham angle to bring Malorie that familiarity and tie her arc together.
Bird Box ends on a hopeful note, but they are still living in a world with these unseen monsters around. How will they have enough food to feed everyone? Do they have to stay there forever? Is there a reason why the monsters showed up, or a cure to make them go away?
There are also the insane people who keep trying to force survivors to take off their blindfolds and see the creatures. Blind survivors can't see the creatures, of course, but Malorie and the kids were just blindfolded, so they may still be faced with that danger if they ever leave the sanctuary.
People asked Susanne Bier about that, and she deflected, saying they only just finished this film and they want to enjoy it for now. But don't be surprised if that surprise ending leads to a new beginning for this story -- with or without Sandra Bullock's Malorie as the lead character.