Lockup is the fifth episode of the fourth season of the television series Agents of S.H.I.E.L.D. and the seventy-first episode overall.
As Robbie Reyes struggles to control the Ghost Rider; S.H.I.E.L.D. infiltrates a high-security prison to unravel the secrets that haunt them all.
In a flashback, Joseph and Lucy Bauer continue their search for the Darkhold. After entering in an abandoned house's basement, they finally find the infamous Book of Spells. They start reading it and realize that the Darkhold displays what the reader wants to read, somehow reading in the reader's mind.
In present time, Phil Coulson and Alphonso Mackenzie visit Joseph Bauer in his hospital room as he got out of coma in order to retrieve the Darkhold as well. Joseph Bauer, terrified by them as he sees them with a demonic appearance, eventually dies after telling her that his wife Lucy has already found the book. She tries to read it in order to abandon her ghostly nature, but the Darkhold shows nothing to her and she screams in despair.
Aboard the Zephyr One, S.H.I.E.L.D. discuss their next move to find the Darkhold. They decide to pay Eli Morrow another visit in prison. Robbie Reyes wants to come with them, but Coulson refuses, much to Reyes' anger.
At the Playground, Leo Fitz tries to reconcile with Jemma Simmons after their argument about Aida's nature. Simmons is worried because she has to undergo a lie detection test and fears that her secrets about Aida and helping Johnson could be discovered. Fitz offers to help her training but she refuses and sends Fitz join the other S.H.I.E.L.D. agents.
Coulson and Melinda May go the prison where Morrow is serving his sentence. They meet with the director in order to complete Morrow's transfer into S.H.I.E.L.D. custody. However, it soon turns out that the director and the rest of the staff has been infected by Lucy Bauer and her acolytes. Therefore, Coulson and May are being attacked by the staff who think they are demons. With Coulson and May locked up in the director's office, Mackenzie decides to send some reinforcements to the prison. He enlists the help of Johnson and Reyes and together they assemble a team. Fitz provides them with a cure for the ghostly infection and remains on the Zephyr One to ensure communications. S.H.I.E.L.D. splits up in two teams, with Johnson going for Coulson and May while Mackenzie and Reyes look for Morrow, who is also targeted by Bauer.
At the prison, Reyes meets Santino Noguera, a gangster who could be responsible for the shooting of Reyes and his brother Gabe. However, Mackenzie stops him from killing Noguera and they keep looking for Morrow. In the meantime, Johnson manages to rescue Coulson and May and the three of them heads to Morrow's cell. They are soon confronted by Bauer who blocks the passage and releases some of the prisoners in order to stop the S.H.I.E.L.D. agents. Fitz manages to open a door so that his colleagues can carry on, but it also means that all doors in the prison are open, including the high security cells.
Mackenzie and Reyes are confronted by Hugo and Vincent, two of Bauer's colleagues. Reyes manages to destroy them and Mackenzie, having been infected by Vincent, injects himself with the antidote in time. They find Morrow and take him out of his cell. Meanwhile, Johnson decides to stay behind and fight the prisoners, who are being recruited into the Watchdogs. She manages to beat several prisoners before being taken down but she is finally saved by Coulson and May who came back for her.
Reyes and Eli make their way through the rioting prison but Reyes cannot resist to confronting Noguera once again in order to discuss about the shooting which crippled Reyes' brother. Noguera protests and says that he had nothing to do with the matter, but Reyes cchooses to kill him anymay because of his gangster activities and turns into his Ghost Rider form to brutally murder Noguera. However, by doing this, he let Morrow walk alone, who was captured by Bauer. Bauer intends to use Morrow to read the Darkhold in her place so that she can be healed of her nature.
At the Playground, Simmons undergoes her lie detection test. When asked whether she had concealed sensitive information, the machine detects that she is not at ease with the question, but before she can answer, Director Jeffrey Mace summons her to his office. Indeed, he need Simmon's help in order to debate with Senator Ellen Nadeer, who is fiercely hostile towards Inhumans and S.H.I.E.L.D.. Mace decides that he will wear an earphone during the debate so that Simmons can provide him facts and figures about the Inhumans.
While debating with Nadeer about the possible threat posed by the existence of the Inhumans, Mace reveals that he is an Inhuman himself. He also mentions the incident in Vienna that made him a national hero when he saved a woman's life. However, Simmons figures out that he is lying on the matter, and when she and Mace discuss the debate with Nadeer in Mace's office, Simmons uses this secret to blackmail Mace in order to be exempted from the lie detection tests.
Back at the Playground, May and Johnson discuss the latter's behavior. May tells Johnson that her efforts for staying away from the team were pointless since Coulson cares too much about her and thus will never give up on her.
Director Mace and Nadeer have a secret meeting during which Nadeer confronts Mace with footage Ghost Rider in the prison, saying that he is a murderer working with S.H.I.E.L.D. Nadeer intends to use this footage in order to blackmail Mace.