In the flashback to the episode "Only the Lonely", it is revealed that Nick Knight and Natalie Lambert met after he attempted to stop a robbery and had a pipe bomb thrown at him. He was severely injured—damage that would have been fatal to a human being—and his supposed corpse was taken to the morgue in a body bag.
As the bag was lying on the autopsy table, waiting for the body inside to be removed for examination, Nick began to heal. The blood that had leaked out was resorbed, and his wounds closed up. When Dr. Lambert opened the bag, she was surprised to find the body so little disfigured.
Before she could begin further examination, she was interrupted by a telephone call. While her back was turned, Nick woke up. Since she had apparently seen a corpse return to life, he had to explain his true nature to her to calm her down; but it was his intention then to hypnotize her into forgetting what she has seen. When he tested her amnesia a few days later, however, he discovered that she remembered everything. He did not have to resort to other means to keep his secret, for she offered to help him find a cure for his condition.