In it the Simpson family visit the UK.
Before Christmas
It's expected to air in the US in November or December of this year. There's no word yet on when it will appear in the UK.
JK appears as herself in the episode, which takes place in the UK.
It's been reported that the Simpson family visit England to find a girl Grandpa had a wartime fling with.
A spokeswoman for JK has already confirmed to CBBC Newsround's website that she'd already recorded her voice for the part.
But the animation takes a lot of time and is still being worked on.
Dozens of stars from The Red Hot Chilli Peppers to U2 have appeared in The Simpsons.
Two years ago, there was a special Harry Potter Hallowe'en segment of the show, with Bart and Lisa going to Springwarts School.
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