The Doctor’s companion Jo Grant, who was concussed when she jumped out of Bessie in Part 3, wakes up in her bedroom at the Cloven Hoof pub during the latest earth tremor that signals the appearance of the Daemon. Before the local doctor sedated her, she was insistent on going to the cavern under the church in search of the Master. Now that she’s conscious again, it’s the first thing on her mind.
Her UNIT friends, Sergeant Benton and Mike Yates, along with Miss Hawthorne, are right at the foot of the stairs, blocking her exit. So Jo sneaks out instead, climbing out of her window, walking across a flat part of the roof, and finding a handy ladder to get down so she doesn’t have to make another dangerous jump.
I thought for a moment that she was intending to hijack Bessie, parked nearby, but she only slips around the car on her way to the church.
The Doctor, meanwhile, is still at the barrier that encircles the village of Devil’s End and prevents anyone from getting in or out by incinerating them. He’s advising the UNIT technical team on the other side of barrier about how to generate sufficient electrical power to supply the oscillator he described in the previous episode so they can create an opening in the barrier big enough to drive their van through.
How to do it? “Reverse the polarity!”
You knew he was going to say that sooner or later
Jo was right: The Master is in the cavern, just where we saw him last, in danger of being trampled by the Daemon he summoned up. He pleads for Azael to go away. But Azael doesn’t go, demonstrating that–in spite of the Master’s claims of commanding the Daemon–he has no control over Azael.
Azael quickly disabuses the Master of that idea, informing him that, while he did come when he was called, that was his decision. “I am not your slave and you are not immortal,” he says rather ominously.
For the first time, we see the giant Azael as he towers over the Master, or at least his furry goat legs and cloven-hoofed feet. There’s a bit of a matte line around the legs, but it’s nevertheless an impressive visual effect.
Once he realizes that he has no power over Azael, the Master tries to make a humble request. “Give me your knowledge and your power so I can rule over these humans.”
But Azael isn’t terribly impressed with him. While he acknowledges that the Master is more intelligent that the human beings around them, he’s also aware that there’s another member of the Master’s species nearby. He will choose one of them, but he hasn’t made up his mind yet which one it will be.
“I shall appear but once more,” Azael says after they’ve discussed the matter, but he warns the Master that he doesn’t like to see failures–and he suspects that the Master’s plans have “Failure” written all over them. Daemons destroy their failures. “Remember Atlantis!”
The amusing thing is that, when he’s faced with Azael, the Master makes the sign of the Horned One. It is an actual, ancient occult gesture to symbolize the Devil, but to modern eyes, it declares that the Master is a total metal-head.
Rock on, Dude!
The Master is deferential to the point of obsequiousness throughout their conversation, but once Azael vanishes in another blast of heat–which the Master flees the cavern to escape–he looks rather pleased with the outcome of this interview.
When he speaks of it shortly afterwards with his henchman, Thorpe the village mayor, he says that it was a mistake to face the Daemon alone. He’ll need all the power he can get to confront Azael the next time. He’ll need the full coven.
By this time, Mike has noticed that Jo is gone, but he knows exactly where she went.
He catches up with her in the cavern after the Master and Azael. 
There are booby traps all over the place to keep out such intruders, but the young couple manages to dodge them. Bok is there as well, napping at the moment in his solid stone form at the base of the pillar.
When they hear someone coming, Mike and Jo hide ineffectively behind a spikey spoked wheel. The Master doesn’t notice them as he sets up his altar for the next ceremony, including a spiffy looking dagger with a red gemstone on the handle.
Back at the Cloven Hoof, Benton and Miss Hawthorne are waiting for word from the Doctor. Benton is getting anxious and restless, since they haven’t heard from him in some time, plus he can’t reach the Brigadier on the walkie-talkie. 
Miss Hawthorne, on the other hand, has made tea and settled down patiently with her knitting while she waits.
At last, something happens, although it’s not what they were expecting. Outside on the village green, the May Day revels have begun. There are Morris dancers, other men and woman dancers for the May Pole, a guy playing the accordion, a man in a rocking-horse costume and another with an air-filled bladder he uses to whack people. All traditional folk elements of such revels. Thorpe is dressed in an strange suit and hat made of strips of newspaper, which may be a modern version of some much older costume.
It’s during the middle of this celebration that the Doctor returns to the village, and where the story most resembles The Wicker Man. The Doctor tries to make his way across the green to reach the pub, but the guy with the air-bladder drives him toward the Morris men, who keep dancing and blocking his way with their clubs. In effect, they surround and trap him and before he knows it, Thorpe is pointing a pistol at him.
They take the Doctor to the May Pole, tie him up, and wind all the colorful ribbons around him to bind him to the pole.
The Doctor tries to talk sensibly to them, but Thorpe announces that this man is a witch who practices the arts of black magic. “And you know what you must do with a witch?”
Build a bridge out of him? No. “Burn him! Burn him!”
One of the Morris men stormed into the pub when Benton tried to come out to rescue the Doctor; Benton puts up a good fight against this man, but it’s Miss Hawthorne who settles his hash by cracking his skull with her purse–she keeps a crystal ball in it, which makes a dandy cosh.
Burt the publican has a blazing torch at the ready to set the Doctor on fire, when Miss Hawthorne rushes out to intervene. Do they dare, she asks her fellow villagers, to harm the great Wizard Qui Quae Quod? (Another little Latin in-joke; these are the masculine, feminine, and neutral forms of the pronoun “Who”.)
Thorpe scoffs at the idea that the Doctor has any magical powers comparable to the Master’s, since he’s all tied up, but the Doctor demonstrates his wizardly powers. With a hint or two from Miss Hawthorne, and help from Benton shooting with a silencer from a window of the pub, he shatters a street light and makes a weathervane spin. He then uses the remote control device that’s still in his pocket to make Bessie drive by itself and run Thorpe over. Not that Thorpe is hurt; he gets up again and tries to run off, but Benton tackles him.
Benton is still amazed by the car trick and wants to know how the Doctor did it.
Doctor: “Elemental, my dear Benton.”
Back at the cavern, the Black Mass is underway. There is a black-robed coven present, but since Burt and Thorpe are elsewhere I don’t think it counts as the “full” group. They are reciting an incantation which I am informed by the DVD commentary is “Mary Had a Little Lamb” backwards.
Jo and Mike observe this from their scant hiding place. 
When the Master places a chicken on the altar and picks up the ceremonial knife, Jo can’t bear to stand silent and watch a helpless animal being sacrificed. She rushes out to stop it.
The chicken’s life is saved, but it turns out that chicken blood wasn’t needed to complete the ceremony. Azael has returned, and we finally see his devilish face.
