For me, this was the weakest installment of the season thus far. The concept was clever enough (you had me at “train in space”) but I’m not sure about the execution.
First of all, I just didn’t like the clock; I thought it was gimmicky and distracting. I like the idea of the time limit, I just don’t think we needed to see the ticking clock as a graphic. Looking back on it, it’s amazing that 24 was able to pull off their clock gimmick for so many seasons. I never found it distracting or inorganic to the drama of the series.
I also thought the development of the guest characters was a bit weak. With the exception of maybe the Captain, their deaths had very little impact because I didn’t quite know who they were.
Throughout “Mummy,” I got the distinct feeling that this was meant as more of a suspenseful “slow-burner” that was sped up for time and as a result some of the moodiness that I felt was latent in the teleplay was lost.
Finally, I would have liked to see this rift between Clara and The Doctor play out for a bit longer. The resolution here was way too easy and I think we could have done with a few episodes of them feuding or even apart. They’ve done it before; in Season 5 after the death of Rory, Arthur Darvill didn’t appear for several episodes and this was addressed in the narrative accordingly (I’m aware that Rory wasn’t necessarily “officially” a Companion at this point, but he was none the less a regular feature who was missing from action for several installments). There might have been some benefit to seeing Clara miss her adventures with The Doctor instead of just realizing that she would miss them if they stopped.
One last thing: I like to believe there’s a cut scene where a character asks the creature, “are you my mummy?” That would have just made my night.