This page of The Witcher 3: Wild Hunt guide has a walkthrough of the main quest The Nilfgaardian Connection. The following walkthrough contains tips, for example how to behave during the confrontation at the Crossroads Inn and how to conduct an investigation in the village of Heatherton.
Going to the Inn by the Crossroads
This quest starts automatically after finishing the Royal Audience main quest; more specifically, after meeting all the important characters in Royal Palace in Vizima. You can either immediately leave the palace or examine it thoroughly - you can find many books, play cards with Nilfgaardian Nobleman.
From the world map select Velen - No Man's Land and move to the marking Hanged Man's Tree (M2,1).
Go West towards the Inn by the Crossroads. When you reach it, talk with the Innkeeper.
The meeting will be interrupted by the arrival of baron's men. How you act will have impact on the future quest concerning meeting the baron. You have two main options: you can either provoke the battle and deal with the bandits or choose the peaceful way of solving the problem. If you fight, try to eliminate enemies one after another, help yourself with signs and don't let enemies surround you. If you choose a peaceful solution, you can either tell them that you're a witcher or offer them some drinks (additionally, be polite). No matter how you acted, after the action you must talk to the innkeeper again in order to learn of the whereabouts of Hendrik.
If you fought in the inn, you will face another group of bandits after you leave the building - you can either eliminate them (start with the archer) or quickly leave this place.
Your next destination is Heatherton located west from the inn. After arriving at the village, take down rabid dogs. Then, listen to the conversation with a peasant. After a cutscene, walk to the hut you were pointed at and investigate the corpse (pick all the options from the list one after another). You will find a key. Walk to the room with fur on the ground. Interact with it to uncover a passage to the basement. In the basement you can investigate the poster, but more importantly, you need to find a chest and examine it. Pick all four options from the list.