A Deadly Plot

A Deadly Plot is one of story related secondary quests available in The Witcher 3: Wild Hunt  involving Sigismund Dijkstra and Vernon Roche.

Journal entry
''Upon finding Dijkstra mingling among the fugitive mages, then watching him assist them in their flight, Geralt got the sinking feeling the erstwhile spy would want a favor in return. He could not have been more correct, for as soon as the mages had set sail for Kovir's hospitable shores, Dijkstra confessed that he had set in motion a plot to assassinate Radovid and asked the witcher to meet him promptly at the Passiflora.''

Meet Dijkstra at the Passiflora
Follow your map to the Passiflora. It is just east of St. Gregory's Bridge. When you go inside, speak to Marquise Serenity and she will open a secret room for you. Head up the stairs and then speak to Dijkstra.