Opening statements in the trial of Michael McCarthy, the Dorchester man accused of killing his girlfriend’s daughter, 2-year-old Bella Bond, and dumping her in the ocean in the summer of 2015, are expected Tuesday.

The discovery on June 24, 2015, of the little girl’s body in a trash bag on a Deer Island beach set off a months-long international search for her identity, and a composite image of her chubby cheeked, smiling face traveled around the world.

State Police detectives solved the “Baby Doe’’ mystery in September 2015, after the girl’s mother, Rachelle Bond, allegedly texted a friend about what happened, and the friend contacted investigators.

Rachelle Bond is expected to be a key witness in the case. She is the only person who saw McCarthy, 37, allegedly killing her daughter as he put her to bed one night in May or June of 2015. Bond has told investigators that she saw McCarthy “moving his arms quickly in the area of the girl’s midsection’’ after a period of silence. He allegedly stored the girl’s body in a refrigerator before throwing it in the harbor.

McCarthy’s attorney has pushed back hard against the credibility of Bond, who pleaded guilty in February to being an accessory after the fact of murder and to larceny over $250 by false purposes, for collecting her daughter’s benefits after she died.

In exchange for her promise to testify, Bond, who has been incarcerated since September 2015, will be credited with time served and placed on two years of probation after the trial.

McCarthy’s attorney, Jonathan Shapiro, has said that with the plea, Bond was “getting away with murder.’’

Bond has maintained that McCarthy threatened to kill her if she reported her daughter’s death. But McCarthy’s former girlfriend, who spent time with both Bond and McCarthy after the little girl’s death, told the Globe in 2015 that they both seemed fine. Bond, the former girlfriend said, was calling McCarthy “hon’’ and touching him on the leg.

Neither mentioned that she had a daughter.

Legal experts said Bond’s testimony could be tough for jurors to accept. Defense attorney Stephen J. Weymouth likened the case to that of former Patriots star Aaron Hernandez, who was acquitted in April of murdering two men outside a club in 2012. In that case, the star witness was Hernandez’s former friend, Alexander Bradley -- a convicted felon who said he saw Hernandez pull the trigger.

“That was a one-witness case,’’ said Weymouth. “A witness who had a tremendous amount of baggage.’’

But attorney Timothy M. Burke, who spent 10 years as a prosecutor, said Suffolk prosecutors have one important benefit: It is believed that only two adults were in the house when Bella died — Bond and McCarthy.

“Ultimately, it comes down to: Does she present well?’’ Burke said. “Can the jury overcome the obvious benefit she’s gaining by her testimony? And, is it the truth?’’

Evan Allen can be reached at evan.allen@globe.com. Follow her on Twitter @evanmallen.