Urusei Yatsura S01 E178 First Love Again!? A Return to the Past for Lum and Rei!!

"Grab her horns and she might just become your darling!"

Jariten finds part of a love locket Lum and Rei got when they were going together. At a birthday party for Mendo's favorite octopus, Lum tries to hide this fact, especially from Ataru.