A Promised Kiss is the 291th chapter of Urusei Yatsura manga.
A special delivery arrives for Lum. The box is from Pigumo, but Lum doesn't recognize the name. Inside she finds a talking frog who asks for a kiss. Lum tosses the frog out, but then recalls a preschool friend of hers name Pigumo who once got turned into a frog while playing with cursed wildgrass. Back then, she had kissed him to return him to normal, but it seems he has been cursed again. Pigumo the the frog comes back to the classroom and Mendo jokingly tells him that he is even more pathetic that Ataru and has no chance of getting a kiss from Lum. When Pigumo shows everyone a photo of his true self, the girls go wild and Mendo nearly jumps out the window. Ataru refuses to acknowledge the situation so Lum takes Pigumo off intending to kiss him. Ataru decides if Lum can kiss someone then so can he. Unable to find anyone, Ataru sits and watches with agony as Lum kisses the frog who changes back into Pigumo though he neglected to tell everyone he is only a few inches tall. Source: Furinkan.com