Toudou Haruhiko is a obese NEET who got summoned to a different world when he was about to get hit by a truck. Unfortunately the summoning system malfunctioned due to exceeding the weight-limit and he was sent back to his world with the summoner Princess Shella.
He retained some Lv.1 skills, but one skill's level got corrupted into Infinity. So he got a powerful version of the skill Judgement, embodied as the cute girl Infinity-chan.
She tells them, that Shella can return to her homeworld, if he looses some weight, so that she can re-cast summoning to open a portal.

(Source: MU)