En route back to UNIT HQ the doctor and Sarah-Jane Smith encounter a shock wave followed by the appearance of a jackal-like face that Sarah is convinced is pure malevolence Upon the TARDIS materialization the doctor surmises they have arrived at the correct site but at the wrong time-in 1911 to be exact in a manor house populated by ancient Egyptian artifacts and a high strung Egyptian man named Ibrahim Namin. A man named Dr.Warlock has been investigating Namin,who has managed to take over the home belonging to the family of two brothers: scientist Lawrence Scarman and his archaeologist brother Marcus. Namin,while consulting what he thinks is an Egyptian sarcophagus is confronted by a creature he thinks is "the great one". The creature promptly murders Namin. And,as the doctor and Sarah soon discover it's Marcus Scarman himself who has emerged along with a team of mummy-like servant robots who eventually kill Dr. Warlock as well. Yet something remains amis. Thanks in part to brother Lawrence's radio telescope the doctor is able to trace a signal sent by Scarman to Mars.
He has determined they are facing an an adversary named Sutekh,an ancient alien from a race called the Osirans on whom ancient Egyptians based their mythology who has been seeking escape from a pyramid on Mars to carry out his biological imperceptive to destroy all life in the universe. And the sarcophagus is actually a space/time tunnel through which Sutekh travels. Realizing with the doctors assistance Sutekh plans to destroy humanities future,Sarah Jane goes about helping the doctor defeat Sutekh. First by a failed attempt to sabotage a rocket he is launching to accomplish his mission,than by almost getting possessed by Sutekh as well during which time divided brother Lawrence is also killed by the man he believes is still his brother. It is only after the doctor and Sarah are transported to the pyramid itself,and find their way out of every trap set by Horace to trap Sutekh and his followers does the doctor manage to destroy Sutekh by transporting him far into the future at the time of his own death,through his time tunnel,before departing in the TARDIS with Sarah.
This story is considered one of the all time Doctor Who classics of the programs original run. And it's easy to understand why. In this story,the doctor is faced with a danger that actually comes extremely near to defeating him. Not only that but the human beings involved have to face the fact that much of their mythology was actually based on extra terrestrial incursion. This is accomplished wonderfully not only by the constant presence of Egyptian artifacts but also the two Scarman brothers. Even though he meets his death at the hands of his possessed brother Marcus,it's Lawrence Scarman's faithful devotion to his brother and torment at his loss to him that motivates him to help the doctor. And to trust his story of being a time lord. The concept of mythology representing something otherworldly rather than spiritual,combined with characters whose emotions are very relatable along with the genuine terror the story and settings provide are probably the key factors in this story's success over the years.