Lilac and Gooseberries

Lilac and Gooseberries is the first main quest in the The Witcher 3: Wild Hunt''.

Spoilers:

Geralt's trail following Yennefer leads him to a village near Vizima. Former Temerian land, but currently under Nilfgaard control, Geralt and Vesemir enters the village tavern to begin their search.

Talk with the tavern owner will not yield any result. Make sure to buy some food from her if you are playing on higher difficulties.

Then, you will have three objectives markers. You can now exit the tavern. For those who want "Fists of the Southern Star" achievement make a quicksave here.
 * 1) Two peasants, use Axii on one of them to get minor details and some XP.
 * 2) A scholar, who will introduce you to Gwent.
 * 3) The Man of Glass who will tell you Yennefer talked with Captain "Peter var Something Something" and suggest you talk to the Nilfgaardians.

Three yokels from former cutscenes will corner you. Use Axii option for a XP boost and an easier fight. (For achievement hunters, this is the earliest time you can get the Fists of the Southern Star achievement, and fairly easy fight)

Go to the Nilfgaardian Camp and talk to the captain.

Afterwards, he will point you to the hunter and the herbalist.

Hunter:

Go talk to him and help him hunt down some wild mutts. He will point you to where the Nilfgaardian patrol was annihilated. Follow the trail with Eagle Sense, I mean Witcher Sense and you will get to the top of the hill with a dead griffin body. Geralt will do an anatomy on the dead griffin.

The Herbalist:

Go to the herbalist and talk to her. You can unlock extra options (that gives you some herbs and alchemy stuff) if you have a Swallow potion (you should already have one if you are playing on the top 2 difficulties) with you. She will tell you you can dive underwater for the buckthorn that will be used as bait.

Go to Vesemir for a debriefing and follow him to the field and wait for the Boss Battle. I suggest that you do the side quest to get the Viper school swords that can make the fight much easier. Make sure to get Swallow and Thunderbolt potions ready. Sword oil is optional.

After the cutscene, shoot the griffin with a bolt, wait until it lands, chop it. Don't hit the griffin directly from the front because it can cause some major damage. Use signs when needed.

After it is down to half health, it will escape. Follow him with either Witcher sense or just simply follow him via Witcher GPS. Fight him again, same strategies apply. After it is dead, loot it to chop the head off and hang it on your horse (one of the 2 trophies in the White Orchard area, the other one is from the Noonwraith).

Go to the Nilfgaardian garrison to turn the mission in. Accept the money (300 XP + 300 coins) or turn down the money (500 XP).

Cutscene will reveal that Yennefer was in bed with the Nilfgaardians.

The following section is entirely not true and contains Game of Thrones spoilers: Geralt goes to the Emperor's bedroom and found Yennefer naked in bed. Geralt strangles her with his Witcher pendant and took a crossbow. He encountered the Emperor in the bathroom. After Triss was repeatedly refered as a "whore", Geralt shot the Emperor with the crossbow and left. The very night, he was smuggled aboard a cargoship bound for Zerrikania by the spymaster of the Emperor.

This is the real story: Geralt found that Yennefer was working with Nilfgaardians and they were summoned to meet the Emperor in Vizima.