After being mysteriously drawn off course to 1911 in an old priory where UNIT HQ would be built, the Doctor and Sarah become involved in the attempted return of Sutekh, an Osirian who was imprisoned by his brother Horus in a tomb recently uncovered by archaeologist Marcus Scarman. He returns to the priory, a zombified puppet of Sutekh, who with help of service robots disguised as mummies, create a deflection barrier around the priory and set about constructing a rocket to destroy the pyramids of Mars to free Sutekh.
The Doctor and Sarah rescue Dr. Warlock, a friend of Marcus's who has been shot by an Egyptian, and enlist the aid of Laurence, Marcus's brother. Laurence is an affable fellow, but despite seeing the possessed Marcus, still thinks of Marcus as his brother and not a puppet of Sutekh. Laurence is played by Michael Sheard, a multiple Who alumni and Admiral Ozzel in The Empire Strikes Back. Bernard Archard (Marcus) is effectively terrifying, his evil-looking eyes, curved down lips, and paled face put to good use.
How evil and how much Sutekh hates life is demonstrated in these lines: "The humans, animals, birds, fish, reptiles. All life is my enemy. All life shall perish under the reign of Sutekh the Destroyer." "Your evil is my good. ... Where I tread, I leave nothing but dust and darkness. I find that good!" Gabriel Woolf's sepulchral voice is put to good use here as Sutekh.
Lots of Egyptology comes in, such as Horus's defeat of Sutekh with the help of 740 Osirians. Not so coincidentally, 740 gods were listed on the tomb of Thutmosis III. The answer to that is the wars of the gods (Osirians) entered into Egyptian mythology and the whole of Egyptian culture founded upon the Osirian pattern. The various sarcophagi and artifacts boost the story's theme.
An interesting discussion takes place between Laurence Scarman, Marcus's brother, and the Doctor. He takes Sarah and Laurence to a future Earth, a desolate planet circling a dead sun, which is how Sutekh would leave it. "Every point in time has its alternative. You've looked into alternative time. ...The actions of the present fashion the future." When Laurence asks him if a man can change the course of history, the Doctor says "To a small extent. It takes a being of Sutekh's limitless power to destroy the future." The Doctor is thus a prisoner of moral obligation--until he stops Sutekh, he just can't up and leave.
Funny lines from Tom Baker: "deactivating a generator loop without a correct key is like repairing a watch with a hammer and chisel. One false move and you'll never know the time again." And he panics at Sarah throwing him a box of gelignite, saying, "Sweaty gelignite is highly unstable. One good sneeze could set it off." When he asks the chastised Sarah for detonators or fuses, she can't find any, and mischievously says, "Maybe he sneezed," meaning the owner of the gelignite. We also learn here that he is 750 years old.
A blaring booboo comes when Sarah claims she comes from 1980. UNIT stories generally take place the year the story is filmed. Also, as the Brigadier retired in 1976 (q.v. Mawdryn Undead, this is clearly inaccurate, as a future story in the same season has the Brigadier still working. So Sarah should've said she comes from 1975.
Trivia: at the time of shooting, the property where this was shot belonged to no less a person than Mick Jagger, but before, the house in the story had belonged to Lord Carnarvon, the archaeologist who uncovered King Tut's tomb, so a coincidence there.
Stylish and evenly-paced, with the Egyptology motif a good asset.