Rupture by Theresa Beachman

He’s spent his life outrunning ghosts, but for her he’ll turn and fight.

Finn Jones is no stranger to sacrifice. After doing time for protecting an innocent, the ex-military pilot has found his true calling with the elite black ops team, the Ocean Wolves. He’s content flying solo, his romantic interests limited to sleek machines with wings or purring engines until he meets the brilliant, buttoned-up scientist who makes his blood run hot. She’s all brain and no-nonsense, but from the moment he lays eyes on her, all bets are off.
Dr. Rose Wyndham thrives in her world of cutting-edge marine robotics. But when an underwater research station hidden beneath the Kalahari desert goes silent, she’s thrust into a deadly mission she never wanted. Blackmailed into action, Rose is determined to tackle the challenge and return to her quiet, solitary life. But what she’s not prepared for? The maddeningly irresistible mercenary assigned to protect her.
Compelled to work together, the rugged mercenary with the stormy eyes pushes all of Rose’s buttons, making her question the safety of her heart as much as her life. And when the mission turns deadly and they uncover a secret research project capable of changing the world, Finn’s protective instincts kick into overdrive.
As they race against time to stop an apocalyptic weapon from being unleashed, the heat between them ignites. And with a madwoman on the loose and global fate hanging in the balance, Finn faces his greatest challenge yet: protecting the captivating woman who’s hijacked his heart.

Rupture is the fourth book in the series. Each book is a standalone romance, which follows an overarching story. They can be read in isolation but for maximum enjoyment are best read in sequence.