Seisouhen can be roughly translated as the passing years. The story indeed starts after many years have passed since Kenshin put down his sakaba sword and decided to settle down. There are also other regular characters, such as Yahiko who has grown into adulthood.
Kenshin and Kaoru have married and has a son named Kenji. But peace does not last long. Kenshin once again have to answer his call to defend the peace of Japan.
While Kaoru waiting for his return, the story turns into flashback of her memories: her first meeting with Kenshin and all of their past adventures. Kaoru waits, and waits for her beloved Kenshin.. but it looks like Kenshin might not be able to come back.