John "Finn" Finnegan is a doctor who treated Steffy Forrester, the CEO of fashion house Forrester Creations, after she sustained injuries in a motorcycle accident. Finn found himself visiting Steffy after she was released from the hospital and noticed she was having a difficult time caring for herself and her young daughter, Kelly. Finn agreed to prescribe Steffy one more bottle of pain pills but refused more. Finn acknowledged his attraction to Steffy and offered to stop treating her for ethical reasons, but Steffy wouldn't hear of it.

Finn was surprised when Steffy's protective ex, Liam Spencer, began questioning Finn's motives. But Finn was even more surprised when Liam recruited him and Steffy's father, Ridge Forrester, for an intervention because Steffy had become addicted to painkillers she had gotten outside of Finn's care. With Finn's help, Steffy agreed to go through rehab and took the treatment quite well.

When Steffy came home, Finn arranged for Kelly to be there, which earned Liam's ire. Finn reciprocated when Steffy professed her love for him and was satisfied when Liam returned acknowledging his jealousy and promising to stay out of Steffy's personal life. Later, Finn found it odd when Steffy's brother, Thomas Forrester, who had a history of mental imbalance and obsession with Liam's current wife, Hope Logan, went on a tirade warning Finn that waffling Liam always cycled back around to Steffy eventually; seeing Liam's adversarial attitude about Thomas, Finn began to wonder if Thomas was right, even blowing off the news that Thomas was keeping a mannequin made to look like Hope in his apartment. However, Finn visited Thomas and saw that he was most definitely unwell, later learning that Thomas had kissed the mannequin and collapsed due to bleeding in his brain.

Finn was rocked when he learned that Steffy had slept with Liam, who had thought Thomas had kissed Hope. Discovering Steffy was pregnant and not sure if Finn or Liam was the father, Finn stood by Steffy and hoped her paternity test would prove him the parent. When the test confirmed Liam instead, Finn remained at Steffy's side but became less certain of how he now fit into Steffy's life.

Finn walked into the hospital lab one day and came across Thomas with his best friend Vinny Walker, who had just confessed to tampering with Steffy's test. Finn punched Vinny to keep him from escaping, then learned Steffy wanted to leave town so Finn wouldn't have to endure Steffy carrying Liam's baby. Finn stopped Steffy and happily informed her that he was the baby's father instead, proposing marriage to her on the spot. Finn received a shock when he came upon Vinny lying on a slab in the hospital morgue; someone had run Vinny over with a car and left him for dead. Finn called Thomas to confirm Vinny's identity. Finn later learned that Vinny had thrown himself in front of Liam's car to frame Liam and clear the way for Thomas to have Hope.

Finn and Steffy learned they were having a boy, whom Steffy gave birth to via a water delivery. Finn asked Steffy for an impromptu wedding, but hedged when Steffy said she wanted to meet Finn's parents first. Finn's father, Jack Finnegan, arrived unannounced, and Finn admitted to Steffy he had been adopted. With his adoptive mother, Li Finnegan, also at his side, Finn exchanged vows with Steffy. But when Finn excused himself to the Forrester guest house to call in about a patient, he was approached by a woman claiming to be his biological mother. Excited to finally know his real mom, Finn introduced the woman to the Forresters, only to learn she was Sheila Carter, who had committed numerous crimes against that family, including causing Steffy's mother, Taylor Hayes, to be presumed dead for years after Taylor and Sheila struggled over a gun.

Finn agreed with Steffy's pleas to distance himself from Sheila; when Finn received a text from Sheila, Finn replied with a heart emoji, figuring the lack of further response would discourage her. Finn confided in new roommate Paris Buckingham about his contact with Sheila and was appalled when Jack demanded that Finn let Sheila hold Hayes while Steffy was out of town. Finn gave in, but when Steffy came home unexpectedly and caught Sheila with Hayes, Finn found himself ordered out of the house by Steffy, who declined to file their marriage certificate. When Finn told Sheila to stay away, Sheila fainted, but Finn didn't believe Steffy's contention that Sheila was faking an ailment. After his fellow doctors couldn't find anything wrong with Sheila, Finn again rejected his birth mother, who sent him another text; Finn admitted to Hope that he still wanted to get to know Sheila.

Finn began to wonder about the identity of his birth father, but couldn't get any answers out of Sheila. Finn reeled when Jack admitted that he was Finn's biological dad and not his adoptive dad; Finn turned his back on Jack as a result of the deception. Later, Finn spent Christmas with Steffy, Hayes, and Sheila after Steffy's returning mother, Taylor Hamilton Hayes, allowed Sheila to celebrate with them.

Finn checked in on Sheila and beseeched her not to let him down, but his hopes were dashed when Steffy came to him and reported how Sheila had purposely gotten her stepmother, recovering alcoholic Brooke Logan, drunk to sabotage Brooke's marriage to Ridge. After receiving a cryptic text from Steffy to meet her in an alley, Finn arrived to find Sheila pointing a gun at Steffy. Finn rushed to stop Sheila and ended up taking the bullet instead, bleeding heavily and flatlining.

Finn woke up several weeks later to find himself hooked up to machines in an unfamiliar apartment. Still weak, Finn could barely take in the sight of Sheila and her explanation that she had brought him back to life because his heart had stopped. As Finn became stronger, he pried further information out of Sheila and learned that Li had first smuggled the barely alive Finn out of the hospital. Finn was enraged when Sheila revealed that Li had died trying to outrun Sheila in a car. Determined to escape Sheila's brand of captive care and find Steffy, Finn nearly strangled Sheila and was eventually rescued by Steffy's former father-in-law, Bill Spencer. Finn was flown to Monte Carlo by Bill, where Finn had a romantic reunion with Steffy. Finn was later overjoyed to learn that Li hadn't died, and sobered by the fact that the fugitive Sheila had died during an attack by a bear.

Finn joined Steffy in championing a reunion between Taylor and Ridge, who was disillusioned in his marriage to Brooke Logan. Later, Finn became suspicious about Sheila's demise and had a toe recovered at the attack scene analyzed; he discovered that Sheila had severed her own digit and faked her death. After the fugitive Sheila was caught, Finn shared Steffy's shock when Bill threatened to jail Taylor for shooting him years prior if Finn and Steffy testified against Sheila; Finn was even more furious when Bill added that he would expose Li for sneaking Finn out of the hospital morgue, effectively tying Finn's hands.

Finn later learned that Bill's apparent romance with Sheila was only a ruse meant to entrap her into confessing to a long-ago murder. When Sheila was about to be arrested and suffered a heart attack, Finn ignored Li's demand to let Sheila die and resuscitated Sheila anyway. With Sheila in jail, Finn clashed with Liam, whom he suspected had renewed feelings for Steffy. However, Sheila was released on a technicality, and Finn found himself grateful to her after she saved Kelly from drowning while Finn was supposed to be watching the girl. Finn's growing sympathy toward Sheila drove Steffy away; once Finn convinced Steffy to return, their homecoming was ruined by Sheila's uninvited appearance. After Steffy packed up the kids and travelled to Rome, Finn received lectures from all sides exhorting him to "do something" about Sheila, implying more permanent solutions.

When Finn learned that Sheila intended to marry Hope's father, Deacon Sharpe, Finn indeed demanded that Sheila leave town and take Deacon with her. Finn reported the engagement to an upset Hope, then counseled Hope over her new romance with Thomas, reminding her how Thomas had once tormented her and kept her from learning that her baby daughter, Beth, was actually alive. Finn reunited with his cousin, Luna, and couldn't understand why his mother Li was so against Luna.

With Steffy's grandfather, Eric Forrester, dying from a not-specified-on-screen ailment, Finn took it upon himself to research the patriarch's condition and devise an experimental treatment to save him; with Eric's daughter, fellow doctor Bridget Forrester, at Finn's side, Finn performed the operation and shared his family's relief when Eric ultimately pulled through. Later, Finn took on a patient, Xander Avant, who told Finn that Thomas had been responsible for the death for a Forrester intern, Emma Barber; Thomas had kept from Hope that her baby, Beth, was alive and ultimately ran Emma off the road to keep Emma from telling Hope the truth. Finn felt dismayed when Steffy defended her brother Thomas, but alerted Hope, against Steffy's wishes, about Xander's claim, since Hope had entered into a relationship with Thomas. After Thomas warned Finn not to interfere, Finn privately vowed to keep Thomas away from Hope.

Finn found it a pleasant surprise to receive a visit from his aunt, Poppy Nozawa, and reminisced about the time Poppy had lived with Finn and his parents, approximately a year before Luna was born. It confused Finn when Poppy came to him lamenting the fact that Luna wanted counsel from her unknown father; Finn agreed to be there for Poppy, but didn't know what the matter had to do with him. Meanwhile, after Steffy stabbed Sheila to death in self-defense, Finn had trouble accepting the act and suffered nightmares; Finn ruffled many feathers by insisting that Steffy had killed his mother, as Finn had always considered adoptive parent Li his mother.

Finn was the only one to attend Deacon's memorial service for Sheila besides Hope. But Finn thought Deacon was crazy when he started telling Finn stories about how the corpse he'd seen at Sheila's cremation had had ten toes, while Sheila had only had nine. Finn continued to rebuff Deacon's claims until Deacon revealed that Sheila's greatest nemesis, Genoa City's Lauren Fenmore, had told Deacon that Sheila had a double in Janet "Sugar" Webber, whom Sheila had once tricked into having plastic surgery to look like Sheila. Feeling Deacon's story had merit, Finn helped Deacon search for Sheila based on credit card charges on Sheila's account, and together they found Sheila chained to a wall in an abandoned building. Finn deemed his mother a hero when Sheila explained how Sugar had returned wanting revenge and had planned to murder Steffy while letting Sheila take the blame, which Sheila had protested. Finn assumed Steffy would be relieved that she hadn't killed Sheila, but Finn got an earful from Steffy, who warned him that he couldn't have both Steffy and Sheila in his life. Finn assured Steffy that he was choosing her.

Finn held his ground and declined when Sheila asked him to stand up for her at her wedding to Deacon, but when Finn went to get takeout at Deacon's restaurant, thinking the ceremony was elsewhere, Finn found himself an unwitting guest. Finn had to smooth Steffy's ruffled feathers when she saw a social media post showing his attendance at the wedding. Finn continued to befriend Hope, attempting to ease her increasing migraines with shoulder massages.

Finn was surprised to learn from Li that the estranged Jack had been texting her. Later, when two of Deacon's employees died of identical drug overdoses, Finn agreed with Steffy and Li that there could only be one culprit: Deacon's other employee, Sheila. Finn ignored Sheila's pleas of innocence and recommended that Sheila be arrested. Finn later got a shock when a scantily clad Hope admitted she was attracted to him. Finn chalked the admission up to Hope's renewed feud with Steffy, but when a tipsy Hope kissed him at a party, Finn rebuked Hope's actions. Finn refused to keep the kiss a secret from Steffy, yet his honesty resulted in Steffy walking out on him and refusing to be contacted.

Finn began to worry when he learned that Steffy had missed an important meeting. After Poppy was arrested for the deaths of the men at Deacon's restaurant, Finn agreed to her request to investigate her apartment and find any clues that might exonerate her.