The Doctor is introducing Adric to N-Space (the original universe) when the Tardis is visited by another power entity. Instead of the Black Guardian who is still after the Doctor, it's the Keeper of Traken. The world of Traken is a place so full of peace, anything evil landing on the planet instantly solidifies and rendered ineffective. So it's a Surprise when the Keeper invites the Doctor to Traken to help battle a coming evil.
One such evil landing on Traken is the creature known as Melkur. He's assigned a person in the grove who looks after him, her name is Kassia. Because of the positive influence of Traken, Melkur is unmovable and his influence is reduced to that of producing a few weeds. Slowly though, Melkur begins to influence Kassia. She finally agrees to help Melkur in his evil ways if he helps stop her new husband from becoming the next Keeper of Traken, as the old Keeper is dying.
When the Doctor and Adric arrive, a body is discovered and they are blamed. But Kassia's husband Tremas sticks up for the Doctor and takes the pair under his guidance. Kassia is surprised by this but despite it being her husband, claims he's conspiring with the Doctor. They are thrown in a holding cell but are rescued by Nyssa, Tremas' daughter. The soon discover Melkur's evil purpose. He wants the power the Keeper of Traken possesses.
Melkur begins to speak and claims to know the Doctor. He also gives Kassia information that helps her to become the next Keeper of Traken. The rest of the council are agreed, especially since Tremas is still at large and currently a renegade. As Kassia begins to assume the role of Keeper, she is overcome with power and her body dissolves, only to be replaced with Melkur, who's sacrificed her to gain control as the new Keeper of Traken. He takes great delight in realize the Doctor still does not know who or what he is.
Adric and Nyssa set off to disrupt the power of the Keeper who is vulnerable while he first starts off as the Keeper. But now free, Melkur causes as much chaos as he can and draws the Doctor inside the statue. It turns out Melkur is actually the Master's Tardis and his goal is to use the power of Traken to regenerate his body as he's used his last regeneration. With the help of Adric and Nyssa, the Doctor escapes as does the Master, but not before he uses the energy to quickly steal the body of Tremas, thus gaining one more regeneration.
PLANET OF ORIGIN: Traken
REASON FOR JOINING: She doesn't. The episode ends with Nyssa searching for her father, who has just been consumed by the Master's conscious. However, between this episode and the next she's taken off planet by someone very familiar to the Doctor.
WHO: Master 14
PLANET OF ORIGIN: Gallifrey
REASON FOR JOINING: Obviously he does not join the Doctor, but this the first episode of the new Master. He will play the Master the rest of the classic series.
NEXT: Tardis Experiences, Great And Numerous