Spencer is about learn Toby’s big, bad secret — and brace yourselves, Pretty Little Liars fans, because her reaction will be nothing short of “gut-wrenching,” warns executive producer I. Marlene King.

The revelation occurs during this Tuesday’s installment (ABC Family, 8/7c) — which pays homage to the Stephen King classic Misery — when Keegan Allen‘s alter-ego “makes a slight mistake” in his ‘A’ game, and, King says with a laugh, his girlfriend almost instantly “picks up on it… because she’s smart like that.” Soon, all of the jagged pieces just start falling together.

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“When Spencer finds out, she almost goes through the stages of grief: First, it’s denial, then anger, then sadness and, in her odd way, acceptance, too,” the EP tells TVLine. “Her original thought is to say, ‘Please, Toby, tell me there’s more to the story’ — and we leave you hanging with that for awhile. But she’ll [initially] keep it a secret. That’s her journey through the first half of Season 3B, because she doesn’t want the girls to know — especially when she’s still trying to process and make sense of it.”

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A “cat-and-mouse game” will ensue between the two at first, shares King, but ultimately this killer betrayal will “implode the relationship. It’s the unraveling of Spencer Hastings,” adds fellow PLL boss Oliver Goldstick.

“It’s a gut-wrenching episode to watch,” King confesses. “It’s really good, dramatic fun, but it’s also really sad.”