Sandra Nicholson is an NYPD 911 operator.
When she was 14, Sandra was babysitting a 3 year-old named Joseph. While she was giving him a bath, she went down to get a bath toy he wanted. When she came back up, Joseph had drowned in his bathwater. Following a police investigation, Sandra was found not guilty by the court, but her personal feeling of guilt plagues her.
Sandra became a 911 operator in part to atone for what she did as a child, eventually becoming a training supervisor. When the Machine generates her social security number, Finch goes undercover as a trainee 911 operator to keep an eye on her while Reese and Shaw investigate. One day, Sandra receives a call from a frightened young boy named Aaron, who reports men attempting to enter his home. Sandra instructs him to hide and wait for the police to arrive, but when Aaron's phone emits the low battery tone, he is found by the thugs, who kidnap him.
Minutes later, Sandra receives a call from a mysterious man who sent the kidnappers. He tells her that she must do as he says to keep Aaron alive. He instructs her to turn off a back-up generator in the building, steal a set of keys from a colleague, and access the call center's server rooms. Once she is in the server room, he tells her to delete all the calls made in on one specific day. As Sandra hesitates, Finch joins her, having muted the man's calls, and offers his help.
Following some investigations by Finch, Sandra comes back into the server room and accidentally looks at Finch, causing the caller to spot him. He tells her that Aaron would die and set off a bomb. However, he reveals it was merely in a parking garage and that Aaron is still alive. Sandra is forced to begin deleting the files.
After Reese and Shaw rescue Aaron, the caller learns that the police were involved in the investigation and tells Sandra not to delete the files before she can confirm the deletion. He bids her farewell. Suddenly, the lights in the room go out and the people inside are instructed to evacuate.
Finch, however, knows that the caller's plan is to kill Sandra once she was outside the building. The two go by an alternate route. The hitman goes down to the basement and steps in a puddle of water, when Finch turns on the lights and reveals a wire in his hand. He threatens to use the generator's power to electrocute the water and kill the man if he does not surrender. The man's voice, however, doesn't match that of the mysterious caller, and he is knocked out by Sandra.
The next day, Sandra thanks Finch at the park, and she spots Aaron playing happily in the park. The caller is still on the loose, however, and warns Finch that he'll be after him.