"What Lurks in the Heart" is the 1021st episode of Casualty and the 17th episode of the 31st series. It was preceded by "New Year, New Me, New You" and followed by "Back to School". The episode was directed by Jamie Annett and written by Rachel Smith, and is the first episode to air in 2017. This episode marks the final appearance of Sebastian Grayling, portrayed by Rik Makarem. The events of this episode take place on 1 January 2017.
Alicia and Ethan wake up together on New Year's Day, but are soon startled when Robyn knocks on the door. In the commotion, Ethan ends up tripping over and banging his knee on the radiator. Meanwhile, Dylan wakes up in the mess of his houseboat, and is awoken by Max and Jez who break down the door to his houseboat when they don't get an answer. He is quick to shoo them away, although they say that they're there to help him, and encourage him to go into work and fight his story. Whilst walking to work, Max comes up with an idea: that Jez could record Seb confessing on his phone.
Elsewhere in Holby, a young boy, Sheldon enters a café looking for Neil Ogden. When he notices that Neil has Down syndrome, Sheldon runs out of the café but he chases him. On some stairs, Neil attempts to stop Sheldon, but they both end up falling down and injuring themselves on some broken glass and rubbish at the bottom. The paramedics arrive, by which point Sheldon has made a run for it. However, when Neil alerts them that a teenage boy was there, it's not long before Jez tracks him down and they're both brought into the ED.
Ric Griffin pays a visit to Elle regarding the allegation and the photo. He warns her that if it were to be leaked, the reputation of the hospital would be at stake. She assures him that it's contained within the department. Ethan and Alicia have arrived for work, and she treats his knee injury in cubicles. They narrowly avoid being caught kissing by Duffy.
Meanwhile, another resident of Holby, Lina, slips and hits her head on a kitchen counter at home. Her boyfriend Femi arrives and decides to take her into hospital. Sheldon and Neil also arrive in the ED, and although Neil identifies Sheldon as the boy who was involved in the incident, he tells them not to call the police as Sheldon's his son. It transpires that Sheldon went looking for his father to tell him what he thought at the fact he'd abandoned him when he was young. However, he is left feeling bad when he sees the wider picture.
Alicia and Duffy begin treating Lina, and it soon transpires that she's marrying him so he can get his visa. However, Alicia is preoccupied with her own personal life, and shares a chat with Duffy. She advises Alicia to do whatever is right for her, despite the fact she's in a difficult position. Upon going back to Lina, they discover that Femi is calling off the wedding due to the fact he can't let her marry him for the wrong reasons as she deserves better.
Meanwhile, Seb catches Jez out whilst he's trying to record him, and he goes and tells Elle without mentioning any names. Elle later speaks to Dylan and advises him to go back to his office and instructs him not to speak to Seb. However, whilst they're talking Sheldon sneaks off to confront Neil. When he arrives, he's restrained by Jacob as Neil goes into cardiac arrest. They need to carry out a procedure to open up his chest and perform cardiac massage. Seb freezes, but Dylan is able to talk him through it and Neil is stabilised, much to everyone's relief.
Lily asks Cal how his interview for the South African research project went, but is interrupted when she sees Archie. Although she attempts to arrange another date, he seems disengaged with her and upsets her. David's son Ollie arrives after Rosa has gone to Spain to visit her mother. Noel is finally able to get hold of Grace's medical notes and hands them over to Connie.
Dylan ends up missing his meeting with Elle and Ric, and she warns him to take things more seriously. In reception, Iain brings in the envelope that Neil gave to him at the scene. Upon opening it, Sheldon finds all of the cards that Neil had meant to send to him along with notes. He finally comes to realise that the idea of having Neil as a father might not be so bad after all. Sheldon later goes to visit Neil whilst he's recovering with his carer Kerry.
Shortly after, Seb pays a visit to Elle and admits that he was lying. Dylan is informed and relieved to hear the news. Ethan and Alicia discuss the fact that they'll have to tell Cal, but just as Alicia is about to in the staff room, Cal goes off on another topic. Outside, the two finally prepare to tell him but Alicia backs out at last minute and pretends they were going to discuss Cal stealing Ethan's project. Cal apologises and assures him that they're okay with each other.
Meanwhile, Connie visits Elle and asks her if she'd intubate a patient prior to a scan if they had a suspected brain injury. Elle says yes, at which point Connie asks why she didn't do that when treating Grace. Elle claims that she made a judgement call, but Connie tells her that it may have worsened Grace's brain injury, and that she has no choice but to report her to the trust.
Dylan catches Seb outside the ED just before he goes home, and tells him that he's secured him another rotation at St. James Hospital. Seb apologises again, but Dylan simply encourages him to stand up to his father more, which he does. Max helps to fix Dylan's houseboat door.
Whilst in bed that evening, Alicia gets up suddenly, not being able to spend time with Cal anymore. However, he soon begins to have suspicions when he finds a cuff-link on her bedroom floor.