Stacey Branning has been keeping the key safe for her brother Sean Slater but their mum Jean looks less than pleased to see it in her possession.
Jean is back on the Square to get married and quickly makes it clear she knows the secret behind it.
She and Stacey fight over the key, and she begs her daughter to stop trying to find what the key opens.
The trailer shows scenes set to air next week, but the mystery won't actually be solved until later this year.
It's like Who Killed Lucy? all over again, isn't it?
Executive producer Dominic Treadwell-Collins recently told the Daily Star Online: "Jean knows all about the key – that’s why she reacts like that. When she returns you’ll find out a little bit more.
"And then in the autumn all will be revealed. We like to keep our secrets."
But, just like the whodunnit?, many eager fans have come up with theories behind the key – could Sean actually be Kat Slater's son?
Earlier this year it was revealed that Kat's daughter Zoe actually has a twin brother and, during the same episode, Stacey was on Skype to her brother.
Was the a major clue, or just a red herring? Could Sean's key open a box containing his birth certificate, revealing his true parentage?
It looks like we'll just have to wait and see…