“Requiem” is episode nine of season one of Sanctuary.
This episode opens with Dr. Magnus trying desperately to get out of a sealed room in some sort of submersible, into which she has been trapped by Will, before she apparently dies from lack of air. The episode then goes back to 6 hours earlier.
Will and Magnus are heading into the Bermuda Triangle in a submersible, the Nautilus, to investigate as ‘Sally,’ the Sanctuary mermaid, has apparently received a psychic distress signal from her people. When they arrive there they find over a hundred merfolk all torn to shreds. They’re apparently a peaceful people, so a predator is assumed, but there’s a weird substance inside them. They start returning to the surface, then Magnus starts acting oddly, and starts suffering from decompression, and has to return the submersible to a lower depth. The two events are most likely related – and they are; Magnus has been infected by some sort of parasitic lifeform that also affected the merfolk.
Magnus has been infected, but Will hasn’t. They have to remain submersed, as returning to the surface will kill her, but they keep descending, past the maximum depth of the Nautilus, and they only have a limited supply of air. Magnus tells Will to kill her if he needs to, to save his own life, as she starts behaving increasingly erratically due to the parasite – which led the merfolk to kill each other. It’s obvious that the events at the beginning of the episode were related to this in some way, as Magnus starts becoming a hazard to both herself and Will.
Apart from a brief video chat with Henry, none of the other characters appear in this episode.