Rainey Collins thought she knew who she was-until the lies and secrets of her past began to unravel everything. Now she's barely holding herself together, haunted by memories she can't escape and convinced that the one man she loves will only suffer because of her. Pushing Max away feels like the only way to protect him. Max Cano never wanted the spotlight, but his rising boxing career promises everything he never had growing up-except the one thing he actually wants. Losing Rainey is the one thing he refuses to accept. He won't give up on her. On them. But when someone they both trusted turns out to be their greatest threat, everything changes. Rainey finds herself trapped in a world she never saw coming, and Max has one mission-keep her safe. Through betrayal, heartbreak, and a world that keeps trying to tear them apart, one thing remains true: they have always had each other's backs. This time is no different. **Unbreakable Thread is the second book in the Invisible String duet, part of The Underground Series.