Doctor Who - Season 0 Episode 72 : Terror of the Vervoids: Part Four 1989

With the Vervoids’ threat fully unleashed, the Doctor uses vionesium to force the plants into their natural death cycle under light and oxygen. The final confrontation sees the Vervoids withering and dying; those survivors aboard Hyperion III escape the carnage. The Doctor and Mel depart in the TARDIS, leaving a ship badly shaken but saved—and the moral cost of extinguishing a sentient species unspoken in this version.