Triss - Blindingly Obvious
When you enter Triss' room in the Rosemary and Thyme dialogue with her that begins this quest will start automatically. When she's through taking you'll have the option of fast traveling to this location - otherwise you'll have to travel to northern Novigrad where the Passiflora is. There will be a fountain in the back you'll be using (pictured above).
At the fountain there will be a scene where Triss uses divination to see the next location you'll be heading, which turns out to be Dijkstra's bath house. Follow Triss over and when you arrive the person you're looking for will be busy attacking everyone. After the scene you'll have to kill three of Dijkstra's guards that get mind controlled then follow Philippa down into the sewers.
When you encounter Dijkstra's troll Bart, who has had his mind taken over by Philippa as well. Tell Bart that you wish to help Philippa and he'll let you pass without a fight... However if you tell him you need her then he'll attack you and you'll have no choice but to kill him.
Follow the linear sewer passages until you reach the really large room with Philippa in it (we've been here before). She'll summon a big fire elemental for you to fight which will hurl fireballs at you as well perform a large AoE ground attack. If you use Aard against it you'll put out its flames and prevent it from being able to use its long range fireball spell on you.
Fire Elemental Weaknesses: Dimeritium Bomb, Northern Wind (Bomb), Elementa Oil and Aard
Defeat the Fire Elemental then sneak/run up to Philippa. Try to avoid chains or broken wood which will give away your position and cause her to teleport down below. When you approach her there will be a scene with you grabbing her and subduing her. As you lead her out of this area you'll have another conversation with Dijkstra - refuse to give him Philippa and go through his dialogue options.
Assuming you meet all of the pre-requisites you'll accept Reason of State at this point in time. This quest is one of the major side quests of the game that will effect the game's ending.