The Great Muppet Caper (1981)

Kermit the Frog and Fozzie Bear are newspaper reporters sent to London to interview Lady Holiday (Dame Diana Rigg), a wealthy fashion designer whose priceless diamond necklace was stolen. Kermit meets and falls in love with her secretary, Miss Piggy. The jewel thieves strike again, and this time, frame Miss Piggy. It's up to Kermit and The Muppets to bring the real culprits to justice.