On the way to the Logan International Airport for a flight to Akron, he collapses in his cab, probably due to having too much inderal according to Maura. Brought into the morgue after being announced dead by the EMTs, he gets an erection while being studied by Maura, and Jane who had just called him "hot". Maura grabs his penis and feels a pulse. She realizes that he was announced dead because of an obstruction in his airway. Due to the urgency of the situation, Maura is forced to perform a cricothyrotomy to save him as they wait for the EMTs. She performs the procedure successfully and saves him. Half-conscious, Dennis smiles at Maura before being wheeled away.
Dennis later has flowers sent to Maura as a token of his gratitude for saving his life. He later visits Maura and Jane to thank them. He gives them gifts, particularly a personally sculpted hand for Maura, and he asks her on a date. On the date, Dennis has Maura tag graffiti with him. Frankie later pretends to bust them in front of his sergeant but lets them off the hook later.When they follow up dinner at Maura's house, Dennis strips nude as he prepares his art materials. He then asks Maura to be nude for him so he can sculpt her, and she agrees, but they don't sleep together because... apparently, her bed wasn't made.
Dennis returns in "Melt My Heart to Stone", after three months of not calling Maura because of traveling for the promotion of his bestselling book, "Release YOUR Inner Winner & WIN!", and tries to make amends with Maura. At first, Maura decides that she will not make up with him but ends up not being able to resist the urge to contact Dennis; Jane, however, tries to keep her in check and away from Dennis.
Later that night, he visits Maura and confesses his feelings for her, saying that he left months ago because he was afraid, telling her that he feels like she is his soulmate and he wants them to have the type of relationship his parents have.
On his 36th birthday, he visits Maura at the morgue, hoping to relive the moment Maura saved his life. He then asks her to spend his birthday with him, and Maura agrees to dinner. He later brings her to his apartment, offers her a few drinks and shows her his art.While he and Maura were at his apartment, Jane continues her investigation. After finding inderal in their latest victims, Jane remembers that Dennis had nearly died of inderal overdose. She then breaks the sculpted hand Dennis made for Maura and sees a frozen hand inside, and she finally connects Dennis to Mark Dugan.
Just when Dennis was about to begin working on Maura, the cops arrive at his apartment and he takes out his knife and threatens to kill and die with Maura then and there. After a conversation with Jane, he explains everything and pushes Maura away from him as he jumps down his elevator to his death.
In reality, Dennis Rockmond is Mark Dugan, the abused son of the prostitute, Rachel Dugan. He is then taken from her and became a ward of the state at the age of 5. After moving between foster homes and being abused again, he finally runs away when he was 8. He then finds his mother, who abuses him again before taking him to another foster home.
Years later, Dennis takes symbolic revenge on her by torturing prostitutes who had abandoned or abused their children, burned their feet with cigarette (the way Rachel did to him), removes their hands (so that they can no longer hurt and abuse children), killing them, then sculpting them into "Venus de Milo" Aphrodite statues, and putting them out in public places that cater to children (all the places he had wanted to go to as a child with Rachel). He then finally takes his revenge on Rachel by killing her and keeping her frozen in a cold storage room before wrapping her in a plaster sculpture.
After killing two more victims, he sends a letter to Jane addressed to his "boss", saying that he is about to sculpt his final victim, which later turns out to be Maura, saying that he was going to honor her because she, just like his mother, gave him life; because her healing hand brought him back to life. He had wanted to send out a message to all mothers to treat their children right because they never know who they will grow up to be: a normal person, or a serial killer who will come back for them like him.
After seeing some of Dennis's victims, Jane briefly wondered if he could be another apprentice of her deceased nemesis Charles Hoyt, given the ritualistic nature of the killings in their area, but there is nothing to suggest that Hoyt and Dennis had any connection.