In StarCraft II, “Lab Rat” is the mission that introduces the Zerg campaign in Heart of the Swarm.
Heart of the Swarm follows Sarah Kerrigan after the events of Wings of Liberty. The opening missions take place at a research facility where Kerrigan is studied and begins rebuilding her abilities. “Lab Rat” establishes the expansion's focus on Kerrigan's return to the Zerg and introduces the campaign's biological and evolutionary themes.
“The Evacuation” is an earlier Wings of Liberty mission, while “Back in the Saddle” and “Fire in the Sky” are other Heart of the Swarm missions. These similarly named campaign titles can make mission-order questions easy to mix up.
The mission is notable because it helps transition Kerrigan from a recently restored human state toward renewed Zerg command. Heart of the Swarm also expands the Zerg campaign with an evolution system, allowing players to customize aspects of the Swarm as the story progresses.