One of the standout narratives of season 2 was seeing Daenerys' attempt to enter Qarth and the challenges she faces whilst there. She and her khalasar are permitted entry into the city due to Xaro Xhoan Daxos performing a blood oath to vouch for them. The Mother of Dragons must then curry favour with members of The Thirteen, rulers of Qarth, as she needs ships to allow her passage back to Westeros to attempt to reclaim The Iron Throne that is rightfully hers.
Most of the action that takes place in the Daenerys arc this season is altered from the novels. This allows the show to go a different way and portray Daxos as a manipulative character who tries to take advantage of the young Khaleesi. However, he doesn't anticipate her strong will and this is shown when Daxos, in league with the Pyat Pree, takes Daenerys' dragons to The House of the Undying and assassinates the other members of The Thirteen to become king of Qarth.
This does not happen in the novels Daenerys willingly enters The House of the Undying, taking a potion which transports her there as opposed to her dragons being kidnapped. The way Game of Thrones presents this situation adds more tension there's more at stake and the payoff is a tragic scene which sees Daenerys reunited with her late husband Khal Drogo and their stillborn son Rhaego. The books frequently reference her mourning Drogo, but with the limited time each character can receive on screen this addition was a good reminder and suitably poignant, emphasising the great emotional fortitude she'd shown by batting through the vision to emerge victorious.