On February 21, 2018, Harrison Chase arrived in Port Charles. He had been hired by the Port Charles Police Department, and he was teamed up with Detective Dante Falconeri because Dante's previous partner, Detective Nathan West, had been killed in the line of duty. Dante wasn't pleased to be assigned a new partner, and he gave Chase a hard time in the beginning.

On March 13, Dr. Hamilton Finn was hauled into the police station after Julian Jerome objected to Finn taking a bearded dragon support lizard into Charlie's Pub. Finn was shocked when he bumped into Detective Chase -- Finn's much younger brother. Harrison Chase had kept their father's surname, while Hamilton opted to take his mother's maiden name. It was quickly clear that Finn was not as happy to be reunited with his brother, even though Chase was thrilled. Finn wished Chase well, but he was not interested in pursuing a relationship with Chase.

Chase was hurt, and he didn't understand his brother's estrangement from their family, but he suspected that Finn resented their father remarrying shortly after Finn's mother had passed away. Despite Finn's cold shoulder, Chase didn't give up hope that his brother would change his mind.

On June 11, it was revealed that Chase knew Nelle Benson from their days living in Florida. Chase had been a police officer when Nelle had been the prime suspect in the drowning death of her fiancé Zachary Grant. Nelle had seduced Chase then she reported the inappropriate relationship to avoid a murder charge. Chase accused Nelle of deliberately trashing his career to save herself, but he had gotten a second chance in Boston. Shaken, Nelle accused Chase of stalking her then fled.

Before long, Chase and Nelle's new fiancé -- Michael Quartermaine Corinthos -- conspired to set a trap for Nelle because Michael had figured out that Nelle had been gaslighting Michael's mother, Carly Corinthos. Chase pretended to fall back under Nelle's spell, so she would turn to him for help. As predicted, Nelle accused Michael of abuse and she begged Chase to help her out of her predicament. Nelle promised to run away with Chase and raise her unborn child with him once she was free of Michael, so Chase agreed to help her kill Michael. However, Chase and Michael took steps to turn the tables on Nelle and force a confession from her that would be recorded with hidden surveillance equipment.

On July 27, 2018, Chase and Michael put their plan into motion. As anticipated, Nelle confessed to plotting with Chase to kill Michael. She also admitted that she had murdered her previous fiancé, Zachary Grant. However, things went awry when a desperate Nelle forced Michael's car off the road then fled the accident scene on foot. Nelle gave birth in the woods, and she claimed that her son had been stillborn. Unbeknownst to Chase or Michael, Nelle had switched her healthy newborn son with Brad Cooper's newly adopted son who had tragically died of SIDS. Brad was in on the ruse.

After Nelle was arrested and convicted of a slew of crimes, Chase grappled with guilt because he had felt partly responsible for Michael's loss. Chase was grateful for his brother's support. Finn had slowly opened up to his brother and the two had grown closer. That friendship was tested when Gregory Chase paid his youngest son a visit in early summer. Finn was not pleased by his father's sudden appearance even though Gregory had expressed a desire to work things out with Finn. To Chase's disappointment, Finn remained aloof and wasn't interested in Gregory's olive branch.

In October, Gregory returned to Port Charles. Chase explained to Finn that their father was ill, and Chase wanted Finn to help. Finn reluctantly took the case. Gregory made another attempt to make peace with his son, but Finn wasn't receptive. Chase didn't understand his brother's animosity, and neither did Gregory. A few days later, Finn diagnosed Gregory with Lyme Disease. Finn referred his treatment plan to Gregory's doctor in Berkeley, CA.

Gregory tried to talk to Finn about Chase's mother, but Finn refused to discuss it. After Gregory left town, Chase remained determined to be a part of Finn's life. The brothers continued to bond as they each found someone special in their lives.

For Chase, the lady was a young elementary school teacher named Willow Tait. Dante's wife, Lulu, knew the teacher because Lulu's daughter was Willow's student, so Lulu encouraged Chase to ask the beautiful teacher out. Chase decided to borrow a police department K-9 and attend career day at the elementary school. Chase's presentation was a hit, and it impressed Willow who accepted a date with the young detective.

The relationship blossomed, and they made their relationship official in March 2019. However, Chase sensed that Willow was troubled. He knew that her previous relationship had been toxic, and that she'd given her son up for adoption, but he was certain there was more. He was right. Willow confided that she had been involved in a cult called Dawn of Day, and the leader of the cult was the father of her child. Willow was terrified that Shiloh Archer would learn that she'd been pregnant, especially since Shiloh had relocated from Beechers Corners to Port Charles.

Chase and his new partner, Detective Valerie Spencer, began to take a closer look at the organization that many believed had helped the community through outreach programs. Chase was also determined to protect Willow from Shiloh when Shiloh began to seek Willow out.