Spoiler Alert: This article is meant for people who have seen "Interstellar." If you haven't, go see it first, then come right back here.
So you've seen "Interstellar." Congratulations! I hope it lived up to your expectations.
Right now, you might be feeling something strange in your head. It's kind of like the opposite of understanding, and it feels weird. What you're experiencing is confusion, and considering the myriad revelations "Interstellar" saves for its third act, I can't blame you.
That's why I'm here to help with as straightforward an explanation of the end of "Interstellar" as I can manage. Let's go!
Before we jump to the third act, let's remember Brand's (Anne Hathaway) speech arguing for Edmund's planet because love might be a more scientific factor than we've ever admitted and might be a propulsive force we don't understand. Coop (Matthew McConaughey) laughs off the notion, but he'll take that back later.
Okay, third act time.
Professor Brand (Michael Caine) reveals to Murph (Jessica Chastain) that Plan A — transporting humans from Earth to the new home planet — is a complete sham because the gravity equation required to get the stations into space needs data from within a black hole. Luckily, Coop and Brand are right next to a really big one, Gargantua.
After Evil Space Matt Damon damages their ship, the Endurance, its crew changes their plan from returning home to dropping TARS off in Gargantua and heading to Edmunds' planet. Or that's what Brand thinks is the plan.
At the last second, Coop sends himself and TARS into Gargantua, which is the only way Brand would ever actually make it to Edmunds.
So here is where you might have gotten lost.
At the center of the singularity, Coop finds a space that allows him to see into Murph's room across all of time. He can float around, moving backwards and forwards in time, and affect small changes through gravity, like knocking over books and moving dust. He was Murph's ghost all along.
TARS suggests that They, the mysterious beings who opened the wormhole near Saturn, have allowed Coop to find this place, but the astronaut disagrees. "They" are actually "us" — well, future "us." Some future iteration of the human race (so we survive) made a five-dimensional space that humans can experience, and what brought Coop to Murph's room in particular was his love for his daughter.
And now that they're both in the black hole, the computer can relay the data to Coop, who can then send it to Murph through the watch he left her. That black hole data allows Murph to finish Professor Brand's gravity equation, which makes the enormous space stations we see later in the film possible.
Coop ends up near Saturn and the Cooper Station for reasons that aren't explicitly stated — but I'd guess "love." There, he's able to meet a much older Murph, who tells him to travel back through to wormhole to Edmunds' planet. There Brand has successfully established a camp, despite finding her boyfriend dead for reasons unknown.
I think that should do it.
Did I mix something up? Did you understand something differently? Let me know in the comments below!