This quest is obtained during the main storyline quest Blindingly Obvious if you choose to bribe Dijkstra with information about the Nilfgaardian Emperor Emhyr, rather than sway him by force. He will ask you to assist in the assassination of Radovid the Stern. Completing this quest will affect the circumstances of the game's ending.
Momentuous decisions altering the fate of entire nations are often made under unusual circumstances. For example, when Geralt was asked to take part in the assassination of King Radovid, he was standing in a bathhouse in Novigrad, half-naked and staring at a sweat-soaked Dijkstra. If he agreed to join in, he was to attend a meeting in a warehouse by the docks. There the co-conspirators would hash out the details of their plan to kill the most powerful ruler in the North.
Dijkstra tells the witcher to meet him by a warehouse portside of Novigrad.
Note that the secondary quest Warehouse of Woe will start just beside the warehouse if it hasn't been completed before.
Once you enter, you will hear Dijkstra, Roche and Thaler conversing just above you. Joining the conversation, Dijkstra will reveal his plan on how to lure Radovid out without using Philippa Eilhart; the witcher may accept or decline the plan during dialogue. If he accepts, they plan to lure him to the bridge leading to Temple Isle. As the discussions finish, the screen will pan to an owl eavesdropping on the conversation.
When the witcher exits the warehouse, he is greeted by Philippa, stating that their plan would never work, that Radovid would not fall for it with only words, and gives the witcher a ring, to which she claims Radovid would believe him, or maybe even trust him completely if he presents it to him.
When the witcher goes to Radovid to lure him, Radovid is reluctant to believe it as many others have tried to give him false information about Philippa Eilhart's whereabouts, which he had dealt with, brutally. When Geralt presents the ring, he is surprised, but only for a moment and mobilizes his men to the bridge, taking the witcher with him.
Once arrived, Radovid orders his men to secure the bridge so that no one gets in or out. He then orders Geralt killed while he is without his swords. Fortunately Roche's ambush saves him and Geralt must join the fight to defeat the Redanian soldiers and witch hunters on the bridge.
Geralt, Roche and the Blue Stripes corner Radovid at the end of the bridge, but before they can get to him, Philippa appears out of a nearby house. She blinds Radovid with a powder and stabs him in the back with a dagger, then quickly morphs into an owl and vanishes. Geralt, Roche and the Blue Stripes make a quick getaway before any more Redanian forces arrive.
As the mission is completed, Roche, Ves, Thaler and Geralt gather in the theater to celebrate. When Geralt asks whether Roche will continue fighting against Nilfgaard, Thaler reveals that the conspiracy has been in peace talks with Nilfgaard, and they have agreed to assassinate Radovid in exchange for the withdrawal of Nilfgaard's forces from the Northern Kingsdoms apart from Aedirn and Lyria, in addition to making Temeria a vassal state to Nilfgaard.
Dijkstra, however, has other plans. He says he cannot allow the peace agreement with Nilfgaard to happen, because he wants the Northern Kingdoms to be independent at all costs. With Radovid dead, he plans to become Chancellor of Redania and continue the fight against Nilfgaard
Of course, Dijkstra anticipated that Roche, Ves and Thaler won't just accept this blatant powergrab, and he has his men surround them. He offers Geralt the chance to walk away freely from this, leaving the other conspirators to their fate.
If Geralt walks away, Roche, Ves and Thaler will be killed and Dijkstra will come to power as Chancellor of Redania.
If Geralt chooses to fight, he and the other conspirators will kill Dijkstra's men and then Dijkstra himself, leaving them free to arrange the peace agreement with Nilfgaard.