The Doctor is obsessed and depressed about the Daleks, and Tegan leaving. Turlough tries to comfort him when the pair hear a scream from within the Tardis. As you'll recall, he joined the Tardis several episodes ago, but this is the first time we've seen him since. Kamelion has plugged himself into the Tardis computer databank, and is suddenly mentally unstable. The Doctor has Turlough run an alpha rhythm into the Tardis to calm Kamelion. That's when Turlough discovers the Tardis is receiving a distress signal from a Trion ship, his home planet. Quickly he tries to disable the signal.
Meanwhile on Earth, Howard is an archaeologist who finds an odd item among some ruins. Before he can investigate further he's approached by his step-daughter, Peri. She's bored out of her mind and wants to follow some friends she just met to Morocco. Thinking it's a bad idea, he strands her on his boats so she'll miss her flight. Spotting the strange device, and hoping it's worth money, Peri grabs the object found by Howard and attempts to swim to shore. Halfway there, she develops a cramp and starts to drown. While the Doctor is out, Turlough notices she's in danger and rescues her, bringing her into the Tardis to recover from almost drowning.
In a third location, the planet Sarn is a fire planet where the religious official Timanov believes their god Logar demands a sacrifice. To do so, he tries to convince Malkon into giving the sacrificial order. Timanov believes Malkon is the chosen one because he has a strange triangle marking on his arm. Ironically, it's the same marking as the device Peri jumped off the boat with. Back on the Tardis Turlough notices the symbol and pulls his sleeve up, revealing a similar marking on his arm.
Tracing the signal from the device, the Doctor makes his way back to the Tardis, where he discovers Turlough trying to dispose of the device. The Doctor takes the device away and follows the source back to Sarn. The Doctor is unaware of Peri in the other room until she stumbles out along with Kamelion who is now disguised as her step-dad Howard. Once arriving on Sarn, he and Turlough leave to do some exploring. Kamelion takes control of the Tardis and morphs into the Master. As Kamelion tries to regain control of himself, Peri grabs a component from the Tardis and runs off. once stable as the Master, Kamelion goes out in search of Peri. Hoping to catch her before she finds the Doctor. He corners Peri but she threatens to drop the Tardis component if the Master doesn't keep his distance. Peri calls for Kamelion to show himself once again and while he fights the Master for control, Peri escapes.
Kamelion/The Master finds the Doctor first and he insists to know where Peri is. The Doctor is unwilling to cooperate, despite the Master now having the natives on his side. They capture the Doctor and imprison him. That's when Peri shows up. She tries to get Kamelion to show himself again, but the Masters will is much stronger. He grabs Peri and has the others follow him. He uses them to free his Tardis which has been covered over by falling rocks. He then drags Peri inside to force her to work for him. He brings out a box and opens it up, but what's inside terrifies Peri and she knocks it over. It's a miniaturized Master. At some point prior to the story, the Masters weapon misfired and he was hit by the blast. Peri tries to capture him but he escapes into the Tardis console. Finally he opens his Tardis door, allowing her to leave.
While Turlough discovers Malkon is his brother and calls for a Trion rescue ship before the planet destorys itself, the Doctor realizes what the Master is after. He's attempting to restore himself with the planets blue flames which have healing properties. But while the Master was trapped in his Tardis, he set a trap and when sprung, it once again frees Kamelion of the Masters control. But the control is only temporary and he feels the Master regaining control. Instead of having the Master control him, Kamelion pleads for the Doctor to destroy him, which the Doctor does, using the Masters TCE.
With Kamelion now destroyed, the Doctor now needs to deal with the Master who is being revived by the blue flames. But something goes wrong and the blue flames dies away and the master is engulfed in the harmful flames. He begs for the Doctor's help but the Doctor does nothing. Pleading, the Master says "Won't you show mercy to your own..." then he dissolves, seeming destroyed. Horrified, the Doctor leaves the Mater to his fate where he discovers Turlough is leaving him. He's no longer a political refuge. But already wanting to do some traveling, Peri requests to join the Doctor on his travels. He agrees, but says it's only for three months.
WHO: Vislar Turlough
DEPARTURE LOCATION: Sarn, circa 1984
REASON FOR DEPARTURE: Discovering he has a brother and is no longer a political prisoner, Turlough accepts a ride from a local Trion ship (his home planet), back home.
DEPARTURE LOCATION: Sarn, circa 1984
REASON FOR DEPARTURE: Realizing he's unable to resist the influence from the Master, Kamelion requests the Doctor destroy him.
WHO: Perpugilliam Brown
PLANET OF ORIGIN: Earth, circa 1984
REASON FOR JOINING: After helping the Doctor defeat the Master, Peri requests to travel with the Doctor. She'd earlier been pinning to do some traveling.