Synopsis: A young TIme Lord, Cargan, is making his first "flight" in a TARDIS, along with his robot sidekick, Orb. Due to a power drain, they accidentally land on a backwater planet, in the city of Minotarius. There, they're greeted by the Chief Elder, Orton.
Orton tells the duo that he has an unlimited source of power, sufficient to recharge the TARDIS.
Incredulous, Cargan asks to see the power source, known to locals as "the Great One".
When he does, he's shocked to discover that it uses the energy which binds matter as its fuel. Seeing the peril this causes to the entire galaxy, Cargan sets out to destroy the device, despite the negative consequences this will have for Minotarian society. Orb warns him this violates the non-interference laws of Gallifrey.
Cargan nevertheless presses forward with his plan, but not before hooking the TARDIS into "the Great One" for a quick power-up.
When Orton discovers what Cargan has planned, he attempts to stop the young Time Lord. This is done rather easily when Orton throws Cargan up against the side of "the Great One".
The impact has totally unintended effects. The machine explodes, immediately destroying the TARDIS tethered to it. Orb begins to "die". And when Cargan's hearts stop beating, everything Gallifreyan in the city falls suit.
Cargan thus did achieve his goal of saving the universe, but at the terrible price of his own life.
A narration box at the story's end tells us that Minotarian society found a way to survive without "the Great One".
Notes: A curious thing about this story is the question of Cargan's failure to regenerate. Since Cargan is fresh from the academy, it may be safely presumed he was in only his first incarnation. Though occasionally mentioned as possible on television, it is possibly the first time that a Time Lord was depicted as having actually died. It is hard to know, from the story, just why he failed to regenerate. Was the force of the explosion so great that his body couldn't recover? Or was the fact that his TARDIS was destroyed the limiting factor? Both interpretations are possible, because the narrative is silent on this issue.