In San Francisco during the year 1959, four lost souls find themselves on the same night trolleybus. Among them is Penny, a single mother burdened with regret for working the night shift and leaving her three children alone; Harrison, a hapless wannabe singer who succumbs to stage fright, backing out of a pivotal audition; Julia, who leaves her waitressing job behind in pursuit of her boyfriend John, after rejecting his marriage proposal; and Milo, a small-time thief who has just failed to retrieve a valuable stamp book he conned out of a young boy. Their driver, Hal, becomes distracted by an alluring passenger in another vehicle, leading to a tragic accident where the trolleybus swerves off an overpass, claiming the lives of all on board.
Meanwhile, Frank Reilly is en route to the hospital with his wife Eva, who is pregnant. Just as chaos erupts with the trolleybus crash, Frank narrowly avoids disaster, and Eva gives birth to their son, Thomas, right in the car. In a twist of fate, the spirits of the four passengers become intricately linked to baby Thomas, who alone can see and hear them—unknowingly burdening him with their unfinished business. As time flows, the quartet comes to care deeply for Thomas, who, in turn, grows attached to them. However, as he reaches adulthood, Thomas's parents begin to worry about his fixation with his "invisible friends," leading to concerns over his mental health and thoughts of separation.
Fast forward thirty-four years, and Hal reappears, still operating the trolleybus he was once responsible for. Having been condemned to guide spirits to the afterlife due to his previous mistakes, he now seeks to bring closure to the four souls who have lingered by Thomas’s side, each having left behind unresolved matters: Penny longs to know the fate of her children, Harrison seeks to overcome his fears and fulfill his dream of performing, Julia has yet to express her feelings to John, and Milo wishes to return the stolen stamp album that torments him with guilt. Recognizing their attachment to Thomas, they must encourage him to help them find resolution before they can move on.
Reluctantly, Thomas unravels the missed connections. He tracks down Penny's daughters but learns of the adoption of her youngest child, Billy, who was taken in after her passing. Following Milo's lead, Thomas, using the spirits’ influence, manages to retrieve the stamp album but finds himself in hot water with the police, resulting in his arrest. Harrison then seizes the opportunity to sing at a B.B. King concert through Thomas’s body, leading to yet another brush with the law when the same officer recognizes him as Penny's son.
As tension mounts in his relationship with Anne, Thomas struggles to explain the bizarre happenings in his life, leading to a painful breakup when she can’t understand his truth. In a bittersweet moment, Thomas and Julia pen a heartfelt letter for John, only to discover that he has passed on. Julia comes to realize that her unfinished business isn’t with John but rather helping Thomas avoid her own mistakes with love.
Ultimately, Thomas confronts his demons during a night under the stars, expressing his fears and feelings to Anne. In a moment of deep connection, he gifts her a heart-shaped keyring, symbolizing his trust and new beginnings. As they share a dance beneath the vast night sky, four new stars emerge, shining brightly, signifying that Penny, Julia, Harrison, and Milo have finally found peace.
