The episode opened with a young boy (Harper) and his nanny go missing, DB and Finn report to the scene. 

Just as DB and Finn begin to process the room 3 power generators for the city go down and the entire city is plunged into darkness. Hodges and Henry are stuck in an elevator with some trace evidence that DB needed processed.

With the power out Morgan and Greg begin processing blood samples by hand using an “A,B,O” Test that determines blood type. Using this basic information they were able to determine that the Nanny, who gave birth to Harper were not the same blood type, and she had been a surrogate. 

TC, Harper’s father had been using a medium named Karen to talk to Natalie, who was Harpers dead biological mother, and she had taken Harper after she became obsessed with TC and Natalie and believed herself to be Natalie. Sarah and Nick found Harper alive and well in a closet at Karen’s house and DB found Karen in the pool, where she drown herself just like Natalie had died.