This first episode is all set-up, mainly: there's something wrong at Scarman's house -- a priory in England, with rooms full of Egyptian artifacts but the professor himself nowhere to be found. In his place is an Egyptian named Namin who's running things while Scarman is away. Namin's willing to preserve his secrets at all costs, so there's a chase sequence outdoors with Namin and two mummies (yep, walking mummies -- with huge chests) trying to find the Doctor, Sarah, and a wounded gentleman named Dr. Warlock, who was shot by Namin but only injured, thanks to the Doctor's intervention. It's a nice chase (even if we have Egyptian mummies hunting people down in the English countryside), and it eventually culminates with our heroes escaping (for now) and taking refuge at the house of Scarman's brother, Laurence. There's some stuff there with a crude radio telescope and a signal from Mars that apparently translates as "Beware Sutekh" -- although why this signal is apparently being broadcast in English is never explained. The Doctor is now worried ("If I'm right, the world is facing the greatest peril in its history") and they head back to the priory to see Namin summon Sutekh's servant from inside a rather nice special effect in a sarcophagus, who coolly but quite brutally kills Namin, telling him that Sutekh only needs one servant. The servant then very calmly declares that he brings "Sutekh's gift of death to all humanity." It's a creepy cliffhanger.
There's a lot more of this coldness and callousness on display in part two. Sutekh's servant turns into Marcus Scarman, only Scarman looks like a walking corpse (so kudos to the makeup department on that one). Dr. Warlock is strangled to death by a mummy in Laurence's house, and a poacher who happened to be on the grounds when the mummies set up a forcefield around the place is crushed to death between two mummies in a literal death hug -- though not before the poacher tried to shoot down Scarman, only to see Scarman expel the bullets and suck the smoke into his chest, unharmed (well, I think that's what's happening; it's a really neat use of reversing the footage, but it's not quite clear what's supposed to happen beyond "bullets can't harm him"). But the most striking version of this comes from the Doctor himself, as he shows Sarah the consequences of not stopping Sutekh in 1911:
DOCTOR: 1980, Sarah, if you want to get off.The manner in which the Doctor matter-of-factly shows the result of their inaction is quite callous, but it's also very effective (and looks a lot like it's been inserted to forestall viewer questions about how Sutekh could destroy the world in the past).
SARAH: It's a trick!
DOCTOR: No. That's the world as Sutekh would leave it. A desolate planet circling a dead sun.
SARAH: It can't be! I'm from 1980.91
DOCTOR: Every point in time has its alternative, Sarah. You've looked into alternative time.
LAURENCE: Fascinating. Do you mean the future can be chosen, Doctor?
DOCTOR: Not chosen, shaped. The actions of the present fashion the future.
LAURENCE: So a man can change the course of history?
DOCTOR: To a small extent. It takes a being of Sutekh's almost limitless power to destroy the future.
Laurence is meanwhile having trouble accepting that his brother is dead and that the Marcus we see is just a walking corpse, so when the Doctor tries to set up a jamming device to break Sutekh's control over Scarman, Laurence tries to stop him -- just as two mummies burst in to kill them, leaving us with the rather disturbing image of one of them grabbing Sarah's throat as the cliffhanger...
91 And yes, this is the story where Sarah Jane Smith repeatedly states that she's from 1980. This isn't really the place to get into the whole UNIT dating argument, other than to observe that this is one of the few pieces of on-screen evidence to not gel with an early-70s setting for the Pertwee UNIT stories and so is probably the bit brought up most often for the late-70s theory.