Having traveled back in time to the underwater town before its flooding, The Doctor must confront the being responsible for puppeteering the dead. The answers he seeks, however, may come at a tragic price.
SPOILERS AHEAD!
Before the Flood opens with a quick intro by The Doctor. It seems like we're going to need a crash course in some time-travel physics before the rest of the episode will make any kind of sense to us, so he begins with a hypothetical anecdote about Beethoven. He suggests he went back in time to meet him, but found when he got there, no one had ever seen or heard of Beethoven, even his friends and family. The Doctor even goes so far as to assign homework and tells us to Google the "Bootstrap Paradox", which apparently means sometimes a person or object can still exist, even without being initially created, so long as its done by means of time-travel. He goes on to suggest that with no Beethoven, The Doctor would have to publish the sheet music and concertos himself, thus becoming Beethoven; history remaining intact. This whole scenario leads to one ultimate question: who wrote the music in the first place?
The actual story opens with The Doctor, Bennett, and O' Donnell arriving in 1980, in the same military town before the mysterious flood left it completely underwater. They manage to stumble across the same ship excavated in the future, and find a fully wrapped body within. Shortly thereafter, they meet the pilot of the vessel, Albar Prentis from the cowardly planet, Tivoli, and recognize him as the top hat ghost from the future, now bringing the remains of his emperor, The Fisher King, to be buried on Earth. Realizing the Fisher King is still living after having killed Prentis, The Doctor calls Clara in the future, who proceeds to tell him a new ghost has materialized, his own. The Doctor has never been one to accept damnation, no matter how much proof he has of its certainty. Knowing he can't save Clara if he's dead, he has the only motivation he needs to break some of the Timelords' cardinal rules, and face the Fisher King to save her life, and preferably his own.
The Fisher King proves himself to be completely remorseless in his dealings. A conqueror of worlds to his core, he reveals the message that infects the mind and is broadcast via the ghosts he creates is a beacon to his people. He means to summon an unstoppable armada and enslave the human race. The only thing that normally stops The Doctor from tampering with the fabric of time, even to gently tweak certain outcomes, is what he refers to as the "ripple effect"; one thing put out of place could torment everything around it, completely altering past, present, and future. Weighing his options, he draws the conclusion that a drastically altered future is way more favorable to the future the Fisher King has in mind, and puts his plan into motion. Using one of the fuel cells from the spacecraft, he blows up the nearby dam, and floods the entire town, becoming the reason himself as to why it was underwater in the first place.
Before the Flood was definitely a fitting conclusion to Under the Lake. Albeit a bit confusing, made more evident by the fact that The Doctor felt the need to break the fourth wall to give us a quick lesson in quantum mechanics beforehand, it was very well executed. The entirety of the story relied on the understanding of how The Doctor can precisely manipulate the causal nexus in order to save people before they fall victim to it. It was disappointing that The Fisher King wasn't much in the way of villains, despite his gruesome design and being voiced by the talented Peter Serafinowicz (with his beastly roars courtesy of Slipknot frontman, Corey Taylor!), but this was one of those rare stories that was more about the how than the what. The episode did a great job of contextualizing Under the Lake, as well as giving us a little more fantasy/sci-fi Whovian lore to play with in the future.
