Contract: Devil by the Well is the first witcher contract most players take in The Witcher 3, found on the notice board in White Orchard. A villager named Odolan wants someone to deal with the phantom haunting an old well in a nearby abandoned village. It is about as classic as witcher work gets.
Most ghosts haunt ruined castles or towers, but this one has chosen a humble well. Geralt investigates the abandoned village, reads the clues and works out what he is facing: a noonwraith, the ghost of a woman who died a violent death and now appears at the hottest time of day. To beat her, he needs to find the object that binds her to this world and prepare properly, with Specter oil and the Yrden sign to force her into physical form.
The quest is a great introduction to how contracts work in The Witcher 3: investigate, learn about the monster, prepare, then fight. Speaking to the herbalist Tomira afterwards adds a little more to the story of the woman behind the ghost.