You're talking about The Witcher, the first ever short story about Geralt written by Sapkowski and published in Polish science fiction and fantasy magazine Fantastyka in 1986. Sapkowski wrote it for a contest held by the magazine and won third place. At that time he wasn't even bother about writing anything else about Geralt.
Season of Storms is good but so-called saga (Sapkowski doesn't like to call it that way) is way better.