Read no further if you haven't caught up on this week's Game of Thrones. Spoilers ahead.
Also, this article gives away one of next week's plot points regarding succession.
Now that King Joffrey is dead, you might well be wondering, who is next in line for the Iron Throne? A lot of folks in Westeros were fighting over that very question when he was alive — hence the War of the Five Kings — but what changes now that he's permanently out of the picture?
Not even the person playing the would-be king knew the answer to this question when he was cast for season four. Dean Charles Chapman, a teenager who replaces child actor Callum Wharry, told Vulture at the season-four premiere party that he was surprised to discover that his character Tommen — Joffrey's little-seen younger brother — was next in line for the Iron Throne. "I was like, Whoa!" he said. "I know it sounds silly, but I hadn't read the scripts, and I didn't know what had happened. And then I flew over to Belfast so they could dress me for wardrobe, and the lady said, 'Okay, let's get your crown on.' And I was like, 'Crown?' 'Yeah, you know, you're going to be king!' So I just stood there, looking in the mirror, with this thing on my head, stunned."
Not to get too far ahead of ourselves here, but Tommen, like the actor portraying him, is also stunned to find that he's expected to rule the land, with his grandfather Tywin advising him as Hand of the King. Despite getting an age increase along with the actor substitution, Tommen is still too young to question things. For example, he may not realize that he can select his own Hand of the King as well as the members of the Small Council. (Do you really expect Tywin to alert him to either of those facts?)
But not everyone in Westeros will agree with this line of succession, of course. (Especially folks in Dorne, because by Dornish law, the crown would pass to the next-oldest, in this case, the long-absent Myrcella, conveniently a ward of their state. And what fun they would have pitting brother against sister!)
Either Myrcella or Tommen only works if you believe that that all three Baratheon kids were the legitimate sons of King Robert Baratheon and his wife Cersei. Robert's brother Stannis, for one, does not accept the boys' legitimacy, since they were actually conceived from the Queen's twincestous relationship with her brother Jamie. (It's also in Stannis' best interest, as Robert's only surviving brother, to continue to dispute the childrens' lineage). Unless one of Robert's surviving bastards (remember Gendry?) were to be legitimatized and recognized as Robert's son, then Stannis would actually be the heir to the throne. (Not that Stannis would do Gendry this favor and, besides, Gendry remains on the run).
The Baratheons are not the only family in Westeros who claim a right to rule. Robert won (or stole, depending on your perspective) the Iron Throne during a war in which the Targaryens were overthrown. It was believed that Robert would be a good choice because he at least had a Targaryen grandparent. Someone with a little more Targaryen blood than that has been gathering up an army — and dragons — on a neighboring continent, and she's got her eye on Westeros, waiting for a chance to make a grand homecoming. One problem: Daenerys cannot bear children anymore, so she wouldn't be able to further the line.
Separatist Balon Greyjoy has already claimed the title King of the Iron Islands (much like the late Robb Stark claimed the title King of the North). And if Westeros is not united under one king, then several other options pop up. The unseen Prince Doran Martell (the elder brother of Oberyn, he of last week’s brothel visits) could be in line as King of the South, since Dorne has remained independent. If each of the seven kingdoms has its own king, then Tommen's slice of the pie becomes ever smaller. The only chance he has to unite the land is to step into Joffrey's shoes in every way possible. Because he is so young (but not as young as in the books, where he is 8!), Tommen's hold on the Iron Throne might not be enough without Tyrell support, since the country is not united. Margaery Tyrell is queen but also not — the bedding being an important part of Westeros weddings. Without consummation (and witnesses — the guests who strip the bride and groom), her union to Joffrey can be set aside. Since it was a political marriage to unite the two families, the Tyrells might well insist that the royals provide a suitable substitute groom to cement things. Here's looking at you, Tommen.