Lal was a Vian scientist who, along with another Vian named Thann, conducted a bizarre experiment to determine if the population of a planet in the Minarian star system should be saved from an imminent supernova.
They would base their decision on whether the inhabitants of the planet, who were empaths, would show compassion and self-sacrifice toward other beings. The experiment involved one of the planet's inhabitants named Gem and members of the crew of the USS Enterprise.
After Captain James T. Kirk and Doctor Leonard McCoy were tortured by the Vians, Gem healed them both. Subsequently, the Vians inflicted lethal injuries and illnesses on McCoy. They wished to see whether Gem would sacrifice her own life to save McCoy's life - such sacrifice was their stated criteria for determining whether her people were worth saving. Gem helped McCoy, but he refused to allow her to completely save him at the expense of her own life. Kirk eventually convinced the Vians that Gem's willingness was enough and Lal healed McCoy himself, having deemed Gem's world worthy of saving. (TOS: "The Empath")