S1·E128 Scramble! Recapture Lum!

Episode 1 of a 2-episode serial. Ataru and Lum get into an argument, with Lum going to her UFO to sleep that night. But in her sleep it crashes on Mendou's property, with Lum losing her memory in the process. Mendou, disgusted by Ataru's lack of sympathy about Lum's missing, informs Ataru that she's on his property. He also tells him that if he wants her back, he'll have to go on his property and deal with him and his army. After some nudging by Lum's Stormtroopers, Ataru works together with them to save her.