He was portrayed by the late Steven Scott.
5 years prior to the story, on October 1963 in Berlin, Steed had encountered Kartovski and shot him fatally through the heart.
In the present, Dr. Constantine manages to keep the foreign agent alive on ice, and allowing him to mind control others with telepathy. At the Ministry of Top Secret Intelligence, Kartovski mind controls Harry Mercer by making him curl his hand and messing with his hand writing. He then uses the agent to gun down Frank Compton, who had returned from assignment.
Later, Mercer tries to fight off Kartovski's control and the doctor, Constantine informs him to get to the hospital for treatment. Barnes then gets a "call for Boris" and Kartovski controls Barnes to shoot Mercer dead. Under Kartovski's control, Barnes dismisses his butler, then ambushes Steed. As the enemy agent is about to take full control, Barnes attempts to warn Steed. This convinces another agent, Peter Rooke to shoot Barnes dead.
After Hinnell informed Constantine of Barnes' death, the two then plan to transfer Kartovski's mind to Tara King. With Tara taken hostage, Boris is told to "think" nice thoughts which will make her his puppet. However, Steed manages to stop the procedure and in the fighting, Steed and Hinnell wreck the life support machine, sending Kartovski to his permanent death. But then, Kartovski then tries one last surprise; Possessing Peter Rooke.