Saitama is bothered by a rock in his boot, one that he got from the moon. He then asks Genos how much a moon rock is worth, who tells him one gram is worth 600,000¥. Saitama then hurries to go sell it, but accidentally drops it in a nearby parking lot. Saitama and Genos spend all night, unsuccessfully looking for the rock.
Genos' notes on the spaceship fight mention Saitama bringing back a moon rock.
In the anime, the omake is referenced in the epilogue of the first season. In volume 52 of Genos' notes, among other details of the spaceship fight, it's written that Saitama brought back a moon rock that was later lost as proof.[1]
