They killed the professor so it started to suck.
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I thought they dumped him on one of the parallel worlds which the professor thought was their world of origin and therefore started taking credit for all the work. Wait - this was before the human-eye-eating aliens, right?
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What Hades said. What happened was John Rhys Davies was pissed that the producers wanted to further the process of turning the show into a mindless action show, while he wanted to go back to its roots of being a "what if" show. The producers went "oh well, we don't need the pompous ass" and made the writers kill him off. In a really bad way too. He had his brains sucked out, was shot, then the world he was left on blew up.
Same thing happened to the girl who played Wade, only what happened with her was she got upset that every other cast member got a raise, including Kari Wuhrer who had just joined that season, she asked for a raise too, the producers said no, she said it was her or Kari, so they made the writers sentence wade to a Kromagg rape camp.