[Warning: Season 4 discussed in details]
It was a typical second episode of a season - if you'd had an action packed premiere (which Vikings didn't). 'Kill the Queen' was slow-paced and, apart from some five minutes of actual fighting, it wasn't really 'Vikings'-like at all.
The Mercia/Wessex scenes were some of the best in the episode. Queen Kwenthrith had been overthrown by some nobles and locked in a tower with her son Magnus - who by the way, is Ragnar's son. Aethelwulf sets off to rescue her and his is the only fighting part of the episode. (It was done in, what I felt, a somewhat comic viewpoint though). Queen Kwenthrith eventually gets rescued, after doing some protecting of herself on her own, and delivers one of the best episode lines, when she asks a bloody and tired Aethelwulf "what took him so long".
In Wessex, Ecbert and Judith are left alone, since Aethelwulf left to rescue the queen of Mercia, and begin to rekindle their previous season's affair to some degree. Judith wants things now, and when Ecbert said he wanted her "to be free", she had a choice to do whatever she wanted. Still caught in Aethelstan's charm, she chose to learn how to 'iluminate' and Ecbert promptly saw to it. He brought Prudencius, a Frank monk, to teach her the secrets of the sacred books and gained Judith's faith in him once more.
In Paris, we saw Rollo getting a haircut - so much for plot - and being laughed at by his bride, Princess Gisla. (To be honest, it was kinda sad to see him in such a situation, with him being the warrior he is). He then begins to help the Parisians with plans to prevent a future Viking raid, as he had agreed to do in last season's finale. He interacts, for most part, with Count Odo who, unbeknownst to him, is being set up by Rolland and Theresa (two very small characters in the grand scheme of things, so don't worry if you don't know who they are). Apparently, they want to make Odo look bad in the eyes of the Emperor, for some reason we're yet to discover.
Now in Kattegat, we learn that Floki has escaped - with Helga's help - and a search party expediently goes after him. Ragnar shows kindness to Helga and her daughter by giving them provisions, but when Floki is recaptured he isn't so merciful. Floki ends up imprisoned in a cave, arms outstretched in what seems like a truly uncomfortable position, if you ask me. He stays that way through the rest of the episode, and Gustav Skarsgard, who plays Floki, actually manages to show us all his despair. Great acting for real. Oh, and he still doesn't know, but his daughter died this episode too.
Another recipient of Ragnar's forceful demeanor is queen Aslaug. She (stupidly) accuses Ragnar of unjustly imprisoning Floki, since all he'd done "was kill a christian". But Aethelstan has always been more to Ragnar than a religious person, so he strikes Aslaug and calmly says the best line on this episode: "It's not about religion. It's about loyalty and trust. Something you don't understand." (Nope, she really doesn't.)
All in all, a mild episode. Nothing grand happening here, but a lot of minor stories that, I'm sure, will eventually grow into big power plays. And oh, yes, Bjorn was in this episode too. But between ice fishing and walking in knee-high snow, there wasn't much he did worth mentioning.