Saitama vs. Mosquito was the fight between Saitama and a mosquito.
As Saitama is watering his cactus on the balcony, he is lamenting how powerful he has become and how everything is too easy when a mosquito lands on the back of his hand, which he then swats. The mosquito is seemingly unharmed, much to Saitama's dismay.
Saitama uses his speed to slap the mosquito all around the balcony, but is unsuccessful. He tries again, but he fails to kill it. He proceeds to repeatedly try and smack the mosquito, moving so fast it appears as if he's teleporting. He later chases the mosquito on the streets with a can of bug spray, but that too proved unsuccessful.
After chasing the mosquito and getting some bug spray in his mouth, he meets a young cyborg named Genos. After Genos warns Saitama about the large swarm in front of them, Genos uses Incinerate to destroy the swarm, burning Saitama's clothes in the process and leaving him naked.