Bobby Isidore’s second-degree murder trial in the slaying of 23-year-old Leighton Powe Jr. during a drug robbery began Tuesday with his defense attorney telling the jury he plans to ask them to convict his client — but of manslaughter, not murder.

That’s the charge the prosecution’s star witness, Trenton Johnson, received in a plea agreement that already has seen him testify in the trial of Felix Adams Jr.

Adams, who was convicted of second-degree murder in February, shot Powe in the head on Sept. 7, 2013, after a plan to rob him of marijuana went wrong.

Defense attorney Jim Burke told jurors he agrees with 99.9 percent of what Assistant District Attorney Julie Knight laid out in her opening argument in 22nd Judicial District Judge Allison Penzato’s court.

Isidore, Adams and Johnson all were in on a plan to meet Powe at the Dollar General store on Robert Boulevard in Slidell, but while the victim thought they were going to buy pot from him, their real objective was robbery and the end result was death.

Burke argued that his client, like Johnson, did not know that Adams was going to shoot Powe.

Johnson, who drove the truck used in the crime, will serve 25 years in prison, Burke said, and his actions were “nearly identical’’ to those of Isidore.

Johnson, who was 19 at the time, testified that the plan was always to rob Powe, and the reason for bringing a gun along was “to use it for intimidation, I guess.’’

He testified that he heard Adams and Isidore, who were in the back seat of his truck, discussing the gun as he drove. Adams said he was going to put one bullet in it, Johnson said.

Powe got in the truck’s front passenger seat, Johnson said, and Isidore grabbed his shoulders and chest to subdue him. The plan, he said, was for Adams then to grab the pot.

Powe struggled, however, and at one point, he and Isidore recognized and acknowledged each other by name. Then, he said, Adams shot Powe in the head. “It got really quiet,’’ he said.

At Adams’ direction, he drove to Javery Road outside Slidell’s city limits. There, Johnson said, Isidore and Adams dragged Powe — who was still breathing — out of the car and threw him in a ditch, where he was later spotted by some children.

Isidore walked away, and Johnson said Adams started to do the same. But at his insistence, he said, the gunman stayed with him and helped begin a cover-up that included setting Johnson’s white GMC truck ablaze in another neighborhood and reporting it stolen.

Johnson’s story lined up on key points with the testimony of Powe’s girlfriend, Rachael Payne, and his best friend, Kenny Pere, both of whom were with him when he went to sell the marijuana.

Payne, who began dating Powe as a teenager, was living with him at his parents’ home at the time. The three friends were planning a night out in New Orleans to celebrate the couple’s fourth anniversary, she testified. But Powe said he had to run an errand first, and she soon realized that the errand was a drug deal.

She and Pere, who is serving time for possession of marijuana with intent to distribute, both testified that they quickly realized something was wrong. Pere said he saw the truck, which had dark-tinted windows, rocking and jumped out of his car. He heard a single gunshot and saw a flash as he ran toward the truck and pounded on the driver’s window. Payne also jumped out of the car when she heard the gunshot. But the truck drove away, and the two were unable to catch up.

Johnson testified that someone ran up and banged on his window.

Payne testified that she was able to see the driver, who looked “frantic and sick.’’

On cross-examination, Burke questioned both Payne and Johnson about lies that they told investigators initially.

Payne, who went to the Sheriff’s Office at the insistence of Powe’s father, testified that she didn’t know whether her boyfriend was alive or not. “I was scared for Lee. I didn’t want to be the one to get him in trouble,’’ she said. She was also worried about Pere. For that reason, she initially told investigators that Powe was trying to buy marijuana rather than sell it and that the two were alone.

Payne broke down crying at one point, and Penzato ordered a recess to allow her to compose herself.

Burke also hammered on Johnson’s initial cover-up, including his claim that his car had been stolen and other efforts to conceal his involvement. Johnson testified that he had burned his clothing at the home of a friend’s grandmother and had ridden home wearing only his boxer shorts. He immediately took a shower and burned the shorts, he said during cross-examination.

“You had a lot of blood on you,’’ Burke said.

But Burke said he did not remember. “I felt dirty,’’ he said.

Testimony in the case resumes Wednesday.

Follow Sara Pagones on Twitter, @spagonesadvocat.