A great story has lots of layers, and a rich backstory. When you only see part of the picture, you might reach a snap judgment about what's going on — but then you see events from years ago, that make you rethink everything. Here are 10 flashbacks that turn your understanding of a story upside-down.

1. Severus Snape's flashbacks in the Harry Potter series

Thanks to Albus Dumbledore's miraculous "pensieve," Harry Potter can witness events that happened long ago — and the most startling series of reveals come from the memories of Severus Snape. Not just the fact that Snape was working for Dumbledore all along, but also Snape's tortured love for Harry Potter's mother and his awful bullying at the hands of Harry's father and Sirius Black.

2. Lost, "Walkabout"

Lost was the unquestioned champion of rug-pulling flashbacks — at least, for its first few seasons. But the most startling, and story-overturning flashback, comes in the show's fourth episode, when we discover that Locke was in a wheelchair before he came to the Island and was miraculously healed. And instead of being the rugged explorer and adventurer that we've been imagining, "Colonel" Locke was a sad-sack office-worker whose boss tortured him.


3. Person of Interest, "Prophets"

We've learned a lot over the years about how Harold Finch created the Machine, the groundbreaking artificial intelligence that "sees everything" and reports terrorist threats to the government. In one episode, "The Contingency," we see some startling flashbacks that show how Finch restricted the Machine from caring about Finch's own well-being above anybody else's, forcing it to be dispassionate. But the most surprising, and game-changing, is probably the series of flashbacks in this recent episode, where we discover that the Machine was not the first A.I. that Finch created — and one of the previous attempts tried to murder him in an attempt to win its freedom. This casts the Machine's respect for human life, and its moral code, in a whole new light, by showing them to be incredibly rare.


4. Heroes, "Company Man"

One of the most famous story-recasting flashbacks happens in this episode, focusing on Claire Bennet's father, known as Horn-Rimmed Glasses. Until this episode aired, we thought HRG was just a callous jerk who lied to his family and used his Haitian friend to wipe people's memories. But this episode cast HRG in a much more sympathetic light, showing him trying to protect Claire from his own employers, and depicting his tragic friendship with Claude the invisible man. All of a sudden, HRG's relationship with the Company and his slippery behavior take on a whole new light.


5. Buffy the Vampire Slayer, "The Dark Age"

Another familiar character who looks completely different in the light of new flashbacks is Rupert Giles, the starchy librarian who spends his time as Buffy's Jiminy Cricket. Turns out when Giles was younger, he was a bad boy — known as "Ripper" — a warlock who practiced dark magic, summoning a demon named Eyghon with his friends. With disastrous results.


6. Hellblazer #11

Another hero whose dark past gets revealed, suddenly changing the whole picture, is John Constantine. In the 11th issue of his solo comic, we discover that the cocky, trenchcoat-wearing magician had a tragic incident in his rearview mirror. We learn about the 1979 Newcastle incident, in which Constantine and his friends tried to summon a demon named Nergal to banish another demon that was possessing a small child named Astra. The whole thing went tits-up and Astra got dragged to Hell, leaving Constantine permanently wracked with guilt.


7. Batman: The Killing Joke

How much truth is there to the flashbacks in this comic book about the origins of the Joker? We may never know, but they definitely put the villain, and his relationship to Batman, in what appears to be a permanently different light. We discover that the Joker was once a failed comedian who was desperate to provide for his pregnant wife — until she died in an accident. This leads him to become the Red Hood, a faux master criminal, and take part in a robbery that goes horribly wrong — the Red Hood flees Batman and falls into a chemical waste tank and becomes permanently disfigured and insane.


8. Fringe, "Peter"

This episode-length flashback finally reveals just why Walter Bishop broke the walls between universes, and exactly what happened to his son Peter. Turns out that the Peter Bishop we've been seeing all along is actually the Peter from the Other Side, and Walter's real son died due to an illness when he was a child. Instead of broaching the universes due to hubris, as we'd previously thought, Walter made this choice out of grief and the desire to spare another version of his son the same fate.


9. Watchmen issue 9, "The Darkness of Mere Being"

Alan Moore really was the master of world-changing flashbacks, as this issue proves. Until this point in Watchmen, we'd believed that the Comedian's attempted rape of the original Silk Spectre was their main connection — but a series of flashbacks to Laurie's childhood shows that the story was much more complicated. And in fact, Laurie's mother later had consensual sex with the Comedian, who was Laurie's real father. The unlikelihood and absurdity of this event is the axis on which Dr. Manhattan's change of heart — and the story as a whole — turns.


10. Adventure Time, "Simon and Marcy"

And finally, one of our favorite TV episodes of all time. We already knew, thanks to the episode "I Remember," that the Ice King had known Marceline back in the bad old post-apocalyptic days. But this long flashback, in which we discover that Simon wore the Ice King's crown on purpose — knowing what it would do to him — to protect Marcy, is both heart-breaking and horrifying. And like a lot of the other flashbacks on this list, it turns a previously despised villain into more of a tragic figure.


Additional reporting by Diana Biller and Abhimanyu Das