Lal was a Vian scientist who, along with another Vian named Thann, conducted a bizarre experiment to determine if a planet in the Minarian star system was threatened with destruction from a supernova should be saved.
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, but McCoy refused to allow her to completely save him at the expense of her own life. Kirk convinced the Vians the willingness was enough and Lal healed McCoy himself, having deemed Gem's world worthy of saving. (TOS: "The Empath")