Dr. Alex North was brought to town after Marlena Evans began experiencing hysterical amnesia after a miscarriage. It wasn't long before it became obvious that Alex and Marlena had once been involved as he was the first person she remembered, even though those memories were only brief flashes. Alex' actions were very suspicious, as he was frequently hypnotizing Marlena in his attempts to get him to remember his past, but after her husband John forced the issue again and again, Alex finally admitted the truth -- he was Marlena's first (and still legal) husband! Years earlier, he was recruited by the government not long after they were married for a special OPS mission to examine the effects of stress on both friendly and enemy soldiers. He was shot down over enemy territory and captured, and was eventually declared legally dead. When he was eventually free, he learned that Marlena had experienced a bout of hysterical amnesia after being informed of his death and had no memory of him. He still tried to track her down, but by the time he found her she was married to Roman and they had Sami and Eric, and he didn't want to cause her any more problems so he never told her who he was to ensure her happiness.
Over the years, Alex became an expert in amnesia as a result of what happened to Marlena. When he was contacted by Lexie Carver to come to Salem on Marlena's case, he couldn't resist the chance to see her again, to see if he could help her. He employed a series of techniques including hypnosis, focal points, music and medication, but was unable to bring Marlena's recent memory back to her. After realizing that in order to solve her current problem with amnesia he needed to address the original incident, he decided it was best to reveal their past connection so that she could better heal. His announcement infuriated John, who refused to accept Marlena's marriage to Alex regardless of any proof that Alex could offer, and refused to accept Marlena's decision when she chose to be with Alex instead of him.
John followed Alex and Marlena on their honeymoon to a tropical island. John uncovered evidence that Alex was an abusive husband. After confronting Marlena with the information, John went after Alex. The two men struggled, and Alex fell off a cliff, presumably to his death.