"It was certainly not what I expected," detective Eric Anderson told NBC's Keith Morrison. "It was a little much."

A little much? The two-hour video tape in Lesa Buchanan’s apartment was not for the faint of heart. It was a glimpse through a forbidden window into the wild sex life of an outwardly wholesome young couple.

Koulis: I told her she looked gorgeous, she should be in movies. We were kind of kidding. And she asked me, "Would you like to film this?" And I agreed to it. This video tape was a one-time thing, it was new thing for us.

What remained was the record of a lurid weekend of sex. But a few telling things did happen that were crucial to the case handed over to prosecutor Kim Helper.

Kim Helper, prosecutor: On three occasions in the video, you can see Lesa Buchanan holding gauze to the groin area. And in fact, on one of those occasions, Christ Koulis is saying, "Put pressure on it." Which I think is a very clear indication that he knew exactly what had happened and was involved in it.

If he supplied the drug, if he injected her, he had committed a very serious crime in Tennessee.

Kim Helper: If the person dies because you gave them bad drugs or because you injected them unlawfully with the drugs, then if a jury finds, you're guilty of second-degree murder.

And, given the drug misadventure in Kentucky that had nearly killed Lesa three years earlier, it seemed to the prosecutor there was a pattern to this.

Kim Helper: So when you're looking at the case you're saying, wait a second. It happened once before. It's now happened again. And Lesa Buchanan has died.

So, Christ Koulis became -- in the eyes of police -- a murder suspect. But Koulis insisted he had nothing to do with Lesa's drug death, or with any drugs for that matter. He'd been clean since rehab three years earlier.

Christ Koulis: I was enrolled in a voluntary advocacy program, a five-year advocacy program. And that program required that I take between three and four random urine tests a month.

Out on bail, the doctor tried to return to his medical practice and life in Chicago.

He was devastated not by the investigation, he said, but by the loss of the woman he loved.

Christ Koulis: I recorded on a micro cassette recorder her voice off her telephone greeting.

Morrison: Why?

Christ Koulis: A memory of her. Trying to hold on to her. Trying to hold on to something about her.

While he grieved, he steadfastly denied any responsibility for her drug use or her death.

Christ Koulis: It's sad. It's horrible. But, I was appalled that I’m being held responsible for her actions.

Police searched Koulis' Chicago apartment. There they found lots of medical samples and small syringes. A typical discovery in a busy doctor's home, perhaps.

But they also discovered the very same kind of large syringes that had been used to shoot up the dissolved Oxycodone pills.

Eric Anderson: I think it just reinforced our theory that he was directly involved with her death.

Yet it was just a theory. Police could find no identifiable fingerprints on the actual syringe that delivered Lesa’s last injection.

Even so, Koulis remained in the crosshairs. He found himself explaining the couple's drug addiction back in 2002 when Lesa had had her first brush with death, ashamed he had been the one to start shooting up with Demerol.

But Lesa, he said, was a willing participant, along for the ride.

Christ Koulis: She asked me what it was. I told her what it was. She said, "What did it made you feel like?" I told her. She says, "I want some also."

At first Koulis says he gave her the shots, but it went on from there.

Christ Koulis: She learned how to do it herself. And then she, she was very adept.

Fearful of losing her daughter if her addiction was exposed, he says, Lesa had refused to go to rehab, as he had done. But he insists he did not abandon her.

Christ Koulis: I contacted her mother and a friend of hers, and I told them I think they need to come over and be with Lesa, because Lesa was refusing to go to rehab, and she needed someone around her.

Lesa's family says that's a lie, but on one thing they all agree. Was that relationship volatile? You bet.

Keith Morrison: Why would you want to go to a relationship with a woman who you know is really not that good for you, nor you, probably, for her, since you seem to set each other off?

Christ Koulis: You're probably right, that we probably were not compatible. There's things I saw in her, beneath the veneer, and there was good, and there was creative, and there was wonderful.

But Lesa, said Koulis, was troubled by hidden demons.

For a time, a couple of years after the Kentucky incident, Lesa and her daughter moved into Koulis’s place in Chicago.

But soon afterwards, says Koulis, “I found Lesa in the master bedroom.”

Christ Koulis: And she was trying to inject herself. And I asked her, "What the hell are you doing?" I was furious.

Keith Morrison: Had she told you before that she was no longer using?

Christ Koulis: Yes. I mean, I had just worked so hard to rebuild my life.

Koulis claims he was astounded that she was injecting her own concoction of oxycodone and solution.

Keith Morrison: You didn't introduce her to those crushed pills?

Christ Koulis: No. This was her own new idea.

Morrison: How did she find the syringes and the needles?

Christ Koulis: Those are available. Those are available online.

Koulis says the price for their steamy affair was becoming too costly.

Christ Koulis: I made a mistake in 2002. Everything thereafter was an attempt to get back to it, to prove myself that I was not doing that again, and that I could be trusted to be a physician and rebuild my life.

He says he gave her an ultimatum.

Christ Koulis: “You can stay with me, and I’ll get you help. Or you're out of here. Choose." She left the next day. She left the next day, and she went to Tennessee.

Yet several months later, once again, the long distance romance rekindled. Koulis maintains he never again saw or knew of Lesa’s drug use, but there were signs on a visit a month before her death.

Christ Koulis: I told her family she had marks on her, and that she was clearly using I.V. drugs again.

That was a conversation Lesa’s family says never happened.

Christ Koulis: I also understand that her family had a very hard time accepting that Lesa was an active, I.V. drug addict.

To hear Christ Koulis tell it, Lesa’s death that July 4 weekend was the sad but self-inflicted result of a terrible secret.

Christ Koulis: She basically told me that she was going to do it. That's what she wants to do.

The state of Tennessee was not so sure.

Which is why, on the September 13, 2007, Dr. Christ Koulis went on trial, in lovely old Franklin, Tenn., charged with second-degree murder in the death of Lesa Buchanan.

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