At the Weil residence in Bethesda, Maryland, children are playing in the front lawn sprinklers, squealing in delight. Off to the side, a young girl is having a conversation with what appears to be an imaginary friend. The girl's mother summons her; her name is Harper. Harper explains to her mother that she is still learning the rules to a game. While her mother is on the phone and preparing lunch, Harper searches her playroom for tools. Looking for Harper, her mother finds her in the treehouse and climbs up after her. As part of the "game," Harper instructs her mother to step on an "X" on the treehouse floor. When she does, the mother falls through the treehouse floor and onto the rocky ground.
During her son Henry's baseball game, FBI agent Claire Bennigan is busy cheering him on when her phone rings; it's work. She is being called in to investigate a strange case of a woman falling out of a treehouse with the only witness being the victim's daughter. Getting support from Henry, Claire returns to work at the FBI field office and meets her new partner, agent Jessup Rollins. At a local park, Lena Lawrence watches as her daughter Minx plays in the playground. Conversing with a fellow parent, it comes to light that Lena and her husband Wes are working through their marriage after he was unfaithful to her.
At the FBI office, Claire questions Harper, who tells her that she was playing a game with her friend Drill. While Harper can't see him, she tells Claire that Drill speaks to her through the lights. If she wins the game, they will have "a domination." Harper reveals Drill is looking for a new friend. On the playground, Minx accepts an invitation to play the game. Claire and Jessup observe Harper in the two-way mirror as she draws. When Harper is picked up by her father, Claire looks closely at her drawing. Convinced there is a case, Claire runs after her and Harper reveals there was another boy who couldn’t win the game.
Investigating further, Claire discovers that the incident of an imaginary friend named Drill is not a one-time deal. There was a boy named Jackson Bellings who detonated a bomb that killed him and left his mother scarred, who claimed to have an imaginary friend named Drill. Meanwhile, Wes Lawrence arrives in the Sahara and is shown a military plane crash site with no sign of the pilot. At the hospital, Dr. Maria Benavidez attempts to treat an unidentified man who is brought in with many tattoos and is speaking Arabic. He violently awakens and she attempts to calm him down. When she asks his name, he doesn’t remember – he’s a John Doe.
Claire and Jessup meet with Jackson's mother. Ms. Bellings confides in Claire about the game her son was playing before he was killed. She recalls a man standing across the street at the time, prompting Jessup to call for a sketch artist. At the Lawrence house, Lena questions Minx about the game and she reveals that her "friend" knows about Wes' infidelity. In Africa, Wes is given information on the plane that was found, including information on the pilot. Wes knows him. At the hospital, Dr. Benavidez chats with a calmed down John Doe, who cannot remember anything and says he can't even speak Arabic. The two get a surprise visitor: Harper.
As Lena gets ready for a bath, the lights in the bathroom go out. When she realizes the neighbors still have their power, her lights turn back on. A sleeping Minx wakes up to the sound of a toy that turns on. Drill is there and he has a task for her. Following her toy, Minx hears a computer turn on: she knows what she needs to do. She easily logs into her father's computer, allowing Drill to access all of Wes' government files. At the hospital, Dr. Benavidez discovers John Doe was able to make an escape.
Claire is studying Harper's file and receives a sketch of the man thought to be connected to the Drill incidents. While the sketch is loading, she realizes Harper's drawing is of a magazine photograph of the President. She gets a call from Wes, who informs her they found her husband Sean's plane in the Sahara. When Claire counters that he crashed in the Arctic, Wes also tells her Sean's body is missing. He hangs up abruptly, leaving her with more questions. Turning to her computer, she can now fully see the sketch. It matches the man from the hospital – but Claire also recognizes him as her late husband, Sean.
Video chatting with Secretary Frommer, Wes shows him the findings of the plane crash. When Frommer is astounded that anyone survived the crash, Wes expresses his interest in staying and running tests on Captain Bennigan's plane. Frommer orders Wes to come home; there’s something he needs to know. At the Bennigan home, Henry looks out his window. Hearing the night sounds, he speaks aloud – it seems that Henry is cured. Giving his thanks, he says it will be their little secret, with Henry telling Drill he loves him.