The infirmary in the North Pole base contained four "sleepers" and multiple hidden dream crabs. Shona McCullough attempted to cross the infirmary for an undefined purpose by dancing to avoid thinking of them when the Twelvth Doctor and Clara Oswald explosively entered the room, causing Shona to panic and the sleepers to get up. The rest of the base crew entered with rifles to save them when hidden dream crabs descended from the ceiling and drove them all into another layer of the dream state upon which they were "rescued" by Santa Claus.
Later, Clara returned to the infirmary to fetch the dormant crab and was attacked by it, being forced deeper into the dream.
When the whole group realised they were all still dreaming and the four sleepers in the infirmary were in fact the actual crew, the crab hosts exited the infirmary through CCTV screens, killing Professor Albert in the process. (TV: Last Christmas)