Yes I agree, it was an outstanding episode! I'm not so good at analyzing why it was so incredibly effective, but it definitely was. The plot itself on paper may have resulted in a less gripping episode I guess -- in retrospect, some of it appears rather obvious. But the execution was so brilliant that this didn't even occur to me while watching. Just everything was right: The cuts in the beginning between Billie shot and her previous talk with the Doctor about becoming a companion; that "proletarian" character that turned out to be Simm's Master in the end, the silly humor in between ("mainly the tea!" ), the SF element of different time zones on the ship, Missy's weirdness, the creepy scenery of the hospital, the associations to mental ward horror movies when the Cybermen were built ... just everything blended perfectly together IMO, it was immensely thrilling, not a single second was boring. An amazing creative firework!