The series' fourth episode made a major departure from the 1997 film.

Warning: This article contains spoilers for episode 4 of I Know What You Did Last Summer.

From the jump, Amazon's I Know What You Did Last Summer series had big shoes to fill. The 1973 Lois Duncan book was most famously adapted into a 1997 film starring Sarah Michelle Gellar, Freddie Prinze Jr., Ryan Phillippe, and Jennifer Love Hewitt. The story follows a group of teenagers who hit and kill someone with their car. They then dispose of the body, only to discover a year later that someone knows what they did. And furthermore, someone wants to kill them for what they did.

In the 1997 film — very old spoiler alert here — the murderer ended up being the victim. Sorry, that's confusing. It turned out that the fisherman the teens hit with their car hadn't died like they originally thought. Instead, he returned a year later to get revenge.

Going into the new Amazon series, many wondered if the show would deliver the same twist, albeit slightly modified. In the show, the teens don't run over a stranger. Instead, they hit the twin sister of the driver (both of whom are played by Madison Iseman). And then there's a whole twist about how it's actually not the twin they think it is.

I Know What You Did Last Summer
Madison Iseman on 'I Know What You Did Last Summer'
| Credit: Michael Desmond/Amazon Studios

For four episodes, the teens wonder if perhaps "Allison" isn't actually dead, and maybe she's back to get revenge. But episode 4 puts that theory to rest when Allison's dead body — which is actually the body of her twin, Lennon — washes up next to her sister in the ocean.

"I wanted people to think we were maybe doing the movie," showrunner Sara Goodman tells EW. "Because that would be very interesting if the other zombie-ish twin was out doing things."

But when looking at the larger picture, Goodman knew she wasn't going that route. "I knew who the killer was going in," she says. "So I felt like, 'Nope, I'm taking [the movie ending] off the table.' And hopefully people are going to be like, 'Wait, what?'"

Goodman has heard a number of theories about the killer so far. Some people think it's Dylan [Ezekiel Goodman]. Other's think it's the dad or Clara [Brooke Bloom]. And although they've now taken Lennon off the table as a suspect, all Goodman will say at this point is that "Anything is possible."

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I Know What You Did Last Summer
I Know What You Did Last Summer (TV series)

A year after a deadly car accident on their graduation night, a group of teens are hunted by a mysterious killer.

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