An unconventional sexual relationship fueled by illicit drugs underpins the death-penalty murder trial of a Plymouth Township man that began yesterday in Montgomery County Court.

Mark Patrick O'Donnell, 48, is accused of raping and strangling 14-year-old Ebony Nicole Dorsey on Dec. 7.

The Wissahickon High School honors student had been babysitting O'Donnell's 4-year-old daughter at his home so he could spend the night smoking crack cocaine with Dorsey's mother, Danielle Cattie, 34, of Whitpain Township.

In his opening statement, defense attorney Thomas C. Egan III pointed out that O'Donnell had never denied the killing, even confessing while TV cameras were running. But O'Donnell, who Egan said had been impaired by cocaine, denies any sexual assault or premeditation, Egan said.

Assistant District Attorney Samantha Cauffman said O'Donnell's graphic statements to police indicated that his memory was not compromised.

"He remembers what he did to her," Cauffman said, suggesting that the evidence would demand a first-degree-murder conviction.

Cattie testified yesterday that although she initially had tried to hide her drug use from police investigating her daughter's disappearance, she eventually showed them about 45 small plastic bags of cocaine that she had dumped in a trash can.

"At that point I didn't care about anything except finding my daughter," she said.

Police found the teen's body on Dec. 9 stuffed into a plastic container that O'Donnell allegedly had hidden on a relative's property in Blue Bell.

Describing a relationship with O'Donnell that included drugs, pornography and sexual devices, Cattie said: "I thought I loved him."

The day her daughter was killed, she and O'Donnell had taken drugs and played backgammon, Cattie said. O'Donnell kept a knife in his hand because smoking crack often made him paranoid, Cattie testified.

Cattie said she had allowed O'Donnell to go pick up her daughter because "I didn't want her to see me high." She had seen him function while "high so many times," she said, she had no reason to believe "this would be different."

Last week, Cattie pleaded guilty to endangering the welfare of a child and drug offenses that stemmed from the murder investigation. She will be sentenced at a later date.

Philadelphia authorities have labeled O'Donnell a "possible suspect" in the May 2006 triple slaying of a family at their Port Richmond home, where O'Donnell had been assigned as a private-duty nurse.

Prosecutors expect to finish their arguments this week. Judge William J. Furber Jr. is presiding over the nonjury trial.