Kate and Bob became trapped within Knox County when it was quarantined by the government. For six days they survived on their own, reportedly fortifying two houses. Unfortunately, they ran into a hostile group of survivors and Kate broke her leg whilst fleeing. Bob was able to carry her to a nearby house, which is when the player takes control of him.
The player can choose to go through with the tutorial, which follows Bob as he treats Kate's wound, fortifies the safehouse, and gathers food. Once that has been completed, the house is invaded by a raider. He holds Bob at gunpoint and announces that he is taking over the house and will "clean out the infected trash." When he turns his gun on Kate, the player can quickly run over and beat him to death with the hammer they acquired earlier. Once he is dead, Kate will tell Bob to survive, and the player is free to explore and survive without helping her anymore.
Alternatively, the player can smother her with a pillow found in the same cabinet as the sheet (which is needed for bandages). Before dying, Kate will apologise for being a burden. Once she is dead, the player is free to explore and survive.