With the doctor and Turlough having just left Tegan behind on Earth following their encounter with the Daleks,the android Kamelion has managed to interface with the TARDIS's time/space circuitry-after which the doctor homes in on a mysterious distress call- depositing the TARDIS on the island of Lanzorate on contemporary Earth. Meanwhile an American led archaeological expedition,led by a Professor Howard Foster accompanied by his step daughter Peri have uncovered an unusual artifact with a symbol on it that Turlough discovers matches the one on his arm. Board with the expedition and wishing to travel on her own, Peri has taken this artifact and almost drowns attempting to escape. While waiting for the doctor to uncover the distress call, Turlough homes in on Peri and saves her from drowning. When recovered on the TARDIS,Peri is accosted by a man who appears to be Professor Foster,than the Master. She soon discovers that she is actually dealing with Kamelion,who has again come under the Master's control. The Masters true intent is the planet of Sarn,where the last vestiges of the planets native population are culturally divided over the existence of a deity known as Logar.
Kamelion,acting as a remote adjunct for the Master,brings Peri there with his TARDIS while the doctor and Turlough go there in his. Upon arrival the doctor and Turlough are instantly viewed as outsiders and offered to their god Logar as a sacrifice. Upon his arrival the Master/Kamelion tries to coerce the locals,willing to sacrifice themselves to Logar with the coming volcanic eruption which consistently destroys their civilization. As the doctor soon figures out,the Master is trying to use a life enhancing gas produced by these volcanic eruptions to extend his lifespan due to his regenerative illness. Turlough,having discovered that one of the natives named Malkon shares the same mark on the arm as himself,reveals to the doctor that he is actually a political prisoner from the planet Trion,and that is race have agents on planets all over the galaxy. He manages to use the device set up by their crashed ship to make a distress call. Meanwhile the doctor,with Peri's help manages to trap the Master within the volcano after being revived by the gas. This of course,after the too easily manipulated Kamelion begs to be destroyed and Turlough-offered amnesty by his people,returns to them before Peri asks to travel with the doctor.
Overall this is an absolutely wonderful story that is very thematically involved and densely written. Turlough and Kamelion are basically faced with the same situation here: dealing with the truth of their identity. While Turlough survives and reunites with his own people,Kamelion comes to realize he can only be of help to the doctor if he ceases to exist. The themes of revelation from ones self deceptions also comes to play with the Master,so obsessed with his own preservation he doesn't notice the fear based superstitions of the natives of Sarn he is attempting to manipulate are alternately clashing with his own ambitions. While the doctor attempts to foster the Sarn's superstitions to help them understand reality in his classic negotiating style, he shows much suspicion and even near disappointment at the return of Turlough's opportunistic behavior throughout the story. Peri,in her introductory story as the companion, shows herself to be adaptable and perceptive-if a bit skittish. In the end the story provides a potent means for one companion to transition to another,while at the same time delivering an important message about hope over mistrust.