Doctor Who: Every Doctor's Darkest Moment
11. The Fifth Doctor - Burning The Master To Death
In Planet of Fire, the Master schemed to harness the restorative powers of a rare gas on the planet Sarn, after he accidentally shrank himself. Using the Doctor's ropey robot companion Kamelion to orchestrate events, the Master lured the TARDIS to Sarn and attempted to have his old enemy killed.
Learning of the scheme, the Fifth Doctor euthanised Kamelion before turning his attention to his old friend. As the Master used the gas to restore himself to his natural height and gain a new regeneration cycle, the Doctor turned the gas up. This seemingly burned the Master alive, while he begged and pleaded for his lives.
It's a weirdly violent ending for a silly story about a shrunken Master. As if Honey, We Shrunk The Kids ended with Rick Moranis accidentally locking his shrunken kids in the oven. A very dark moment for the charming Fifth Doctor.