Raynor, our knight in shining armour managed to save and partially restore Kerrigan's humanity at the end of Wing's of Liberty but that is not the end of the story. Heart of the Swarm continue the story of Kerrigan rediscovering her psionic and Zerg-controlling ability.
This much anticipated sequel introduced new units to the three factions and gives the player an in-depth look into the Zerg hive. I have sunk countless hours on the single player campaign. If you enjoy playing Wings of Liberty, then it will be very likely that you are going to enjoy Heart of the Swarm.