SVU's 300th episode begins with a trip down memory lane. In 1999, Delores Rodriguez posts flyers for her missing son, Hector. In 2000, Dolores has set up a vigil for her missing son in front of a wall mural commemorating Hector. David and Laurie Morris, a young couple, pass by the mural pushing a stroller. In 2006, the Morris family passes by the mural again with a young toddler, Wyatt, in tow.
In present day, David Morris walks by the mural alone, heading to his wife's apartment to take his son to a Yankees game. In the subway, a mysterious man in a red hat tells David that he's dropped a 20-dollar bill. While David picks up the money, the mysterious man grabs Wyatt and drags him into the train before David can get onboard. David scrambles to the street in a panic and calls 911.
Benson arrives on scene and immediately remembers the neighborhood. Cragen informs her that the missing boy, Wyatt Morris, is the child of a high-profile real estate developer who owns half the buildings on the West Side. Benson remembers Hector Rodriguez, the boy who disappeared from this same neighborhood 13 years ago. Amaro quickly reminds her that today's missing boy takes precedent over the cold case from her past.
David's ex-wife, Laurie, arrives at the scene and immediately attacks David, blaming him for Wyatt's disappearance. Benson and Amaro decide to separate the quarreling couple. Benson interviews Laurie, who can't offer any useful information. Amaro talks to David, who provides him with a general description of the suspect. Laurie begs Benson to find her son.
Back at SVU headquarters, the detectives sift through conflicting eyewitness reports. Their initial suspicions point to one of the parents. They're currently involved in a custody dispute, and there might be a good motive for one of the parents to stage their own son's kidnapping. After speaking with David, the detectives discover that David and Wyatt stopped by a cafe before heading to the subway.
After interviewing a waitress at the cafe, the detectives discover that a man fitting David's description of the suspect was seen at the cafe around the same time that David and Wyatt were there. Munch points out the similarities between this case and the disappearance of Hector Rodriguez 13 years earlier. Cragen assures the squad that Hector's case is closed.
Benson and Amaro return to Laurie's home to talk to Wyatt's parents again. They're interrupted by a call from Rollins, Fin and Munch. Wyatt was spotted in a neighborhood store earlier in the day. He was seen with a man in sunglasses and red hat, who bought blonde hair dye. The detail sounds suspiciously familiar to Munch; it's the same M.O. that Hector Rodriguez's kidnapper used 13 years ago.
Back at the station, Munch digs through his old files on Hector Rodriguez. The detectives are convinced that they're suspect was also responsible for Hector's disappearance 13 years ago, but so far they have no tangible leads on Wyatt's whereabouts. Munch suggests that they dig into the old files in search of a clue. Benson heads to Hector's mother's home. Cragen orders Amaro to accompany Benson, in order to get fresh eyes on the case.
Benson and Amaro visit Delores Rodriguez in search of a clue. Dolores is understandably upset that her son's kidnapping was never solved. Delores tells Benson that a man who identified himself as a police officer sent her a letter on Hector's 13th birthday, urging her to continue pestering the police about Hector's disappearance.
The cold case detective shows Munch and Amaro the letter written by a volunteer officer, Steven Lomatin. Munch remembers the man as a slightly unstable but completely harmless vigilante who wants to be the hero in missing children cases. Meanwhile, out in the field, Lomatin points Benson and Rollins to Wyatt's baseball cap. They decide to bring him in for questioning, just to be on the safe side.
Back at the station, the detectives contemplate if Lomatin should be considered a suspect. Munch says that maybe he overestimated Lomatin's stability when he dismissed him as a suspect 13 years ago. After all, he was directly (and coincidentally) involved in both cases, having found Wyatt's hat today and Hector's lunchbox 13 years ago. Lomatin agrees to show Rollins his "archive."
Lomatin escorts Rollins and Fin to his mother's attic, where he has assembled a massive crime board tracking missing child crimes over the past 13 years. Fin and Rollins find stacks of evidence related to the missing boys. Lomatin assures Rollins that he bought the items in order to track the kidnapper. Benson talks to Hector's mother about Lomatin, but she feels that he's trying to help when the police won't.
While explaining his research to Rollins, Lomatin points out the correlation between children disappearing and fires in the neighborhood. He suggests that someone is kidnapping children, and then putting their bodies in buildings and torching them to eliminate evidence.
Lomatin seems like a prime suspect, but he has an alibi: he was at a pharmacy buying anti-psychotic medication when Wyatt was kidnapped. Munch feels that, even though Lomatin is clearly unstable, there might be something to his correlation between missing kids and building fires. Rollins discovers that, four days after Hector's disappearance, a nearby building replaced some concrete in their basement. Cragen puts in an order to crack the slab.
Under the cement slab, they discover Hector's body. Benson and Amaro break the tragic news to Hector's mother. Back at headquarters, the detectives discover that Wyatt's grandfather owns the building that Hector was found in. David Morris helps the detectives track down the building manager, Lewis Hoda, who is currently involved in a dispute with David.
The detectives pay a visit to Lewis Hoda in the middle of the night. Lewis assures the detectives that they won't find any evidence, but Rollins discovers some pill bottles prescribed to someone else. It's not what they're looking for, but it's enough to take Lewis in for questioning.
Back at the station, the detectives find a number of parallels between Hoda's activities, the missing children and the building fires. He's looking like a great suspect, but Cragen wants to be sure. He sends Benson and Amaro in to question Lewis. Eventually, Lewis admits to the building fires, but he claims he had nothing to do with the missing kids.
Amaro tricks Lewis into confessing to Wyatt's kidnapping by emphasizing Lewis' feud with David, the son of his former employer. Eventually, Lewis tells the detectives where Wyatt is. The squad heads to an abandoned warehouse. After a frantic search, Benson finds Wyatt alive in a storage room. She returns the boy safely to his parents and heads back to Hector's wall mural to comfort Delores, who is grieving for her lost son.