The basic premise is that Roderich is a mafia-type who runs bars and golf clubs, etc. Kiku is Gilbert’s boy and an assassin type. Though Kiku was sent to kill Roderich, he has fallen for Roderich. Roderich, in turn, has also fallen for Kiku.
The artist notes that there would be other parts of the story, such as how Roderich is arranged to be married with Natalya to improve relations but Kiku invades into Ivan’s territory to stop, how Kiku becomes friend with Vash (who’s a police officer), Roderich fights with Vash but Kiku steps in, Gilbert finds out about their secret love, etc.