I Know What You Did Last Summer Season 1 finale ending explained

I Know What You Did Last Summer - Courtesy of Michael Desmond/Amazon Prime Video
I Know What You Did Last Summer - Courtesy of Michael Desmond/Amazon Prime Video /

I Know What You Did Last Summer Season 1 ended with a killer revealed and a surprising twist. Just how did it all wrap up?

When it comes to horror movies, the killer is always someone the lead character knew. That was no exception on I Know What You Did Last Summer. The Season 1 finale didn’t just reveal the killer and the reason why, but it also brought us a twisty ending that could catapult us into a second season.

Caution: We’re going to get straight into spoilers for the ending of the season. If you haven’t watched it yet, you’ll want to click away.

Who killed everyone on I Know What You Did Last Summer Season 1?

After telling Dylan she was actually Allison, Dylan headed to the cops and told the chief everything. The chief sort of believed him, but not enough to really do anything about it. The idea that Lennon was Allison seemed a little off to her, and it’s not like Bruce was telling her anything or suggesting that his daughter wasn’t the twin everyone thought she was.

However, Margot does believe Dylan. When she finds out that Lennon and Allison’s mom is actually still alive, she goes to the cult compound to find the woman. After finding a dead animal down there, she calls Alison and Dylan.

The two friends head straight to her and they come across the dead animal. That’s not the only thing dead. No, it’s not Margot but Allison’s mom. This time the woman really is dead.

As Dylan is knocked out, Alison goes in search for Margot. Well, it turns out Margot was the killer all along. At the very end of I Know What You Did Last Summer Season 1 (stay for the credits), Margot explains it all. Johnny started blackmailing Margot and so he was the first to die. She explains why everyone had to be killed, including the deputy who had stumbled upon an OnlyFans payment that linked Margot to murders so he had to go.

Kyle helped with the murders when he caught Margot killing Johnny. It also looks like Mei is in on it all at one point, although I don’t hear of that being confirmed.

Why did Margot kill everyone on I Know What You Did Last Summer?

Margot actually wanted Allison to tell her the truth. You see, Margot had figured things out during the fall after Allison kissed her and then ghosted her. Photos on social media weren’t quite right.

Allison keeps up the lie, telling Margot that she is Lennon. It looks like Allison is really starting to believe this lie. Is living as Lennon just easier?

Margot just wants to be loved by Lennon, and Allison wants to be loved by Margot. However, Margot stabs Allison and it looks like she’s going to leave her for dead. When Dylan (who had been knocked out by Kyle) turns up, Margot shares that his DNA is all over the knife. She got that from the used condom she found in Allison’s trash.

When the cops show up, Dylan is holding the gun at Margot and she pretends to be a victim. Allison decides to lie for Margot, telling the cops that Dylan did it. So, he ends up going to prison and Margot gets to go free. Allison gets to the hospital and is saved.

In the end, Margot and Allison, who is still pretending to be Lennon, are together as if nothing happened.

It’s not over on I Know What You Did Last Summer

This is a horror series. More importantly, it’s a season finale for a show that hasn’t been billed as a limited series. That means things have to be left open for a second season. That’s exactly what the I Know What You Did Last Summer Season 1 finale does.

As Margot and Allison pretend that everything is perfectly fine with them, we got to a freezer where Riley’s body still is covered in the gunk that she was found in while in the cave.

Riley’s eyes snap open. So she isn’t dead after all?

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What did you think of I Know What You Did Last Summer? Did you guess that it was Margot all along? Share your thoughts in the comments below.

I Know What You Did Last Summer is available to stream on Amazon Prime Video.