Seducing My Sister's Husband Behind Her Back — Episode 20

As William and Fiona's relationship begins to crumble, Fiona's younger brother, Leon, sees his chance to make a move. He urges the ever-honorable William to get back at Fiona by cheating, and even offers to help him "practice." William still sees Leon as the boy he's looked after since childhood, unaware that Leon has spent years plotting to make him his. But as they cross one boundary after another, William is horrified to realize he's falling for Leon. Consumed by guilt, he tries to walk away, only for Leon to chase after him and finally confess the love he's kept buried for ten years.