"What Lies Beneath" is the 1001st episode of Casualty and the 40th episode of the 30th series. It was preceded by "History Repeating" and followed by "Where the Truth Lies". The episode was directed by Sean Glynn and written by Dominique Moloney.
Jacob arrives at work ready to set off later that morning to give his talk on gun control to an audience including some high ranking members of the board. Elle informs Connie of where he's going, and encourages her to accompany him as a friend for support, which she soon follows through with. Rita arrives for work but when she sees Iain outside the ED, she quickly ducks behind a parked car. However, when she's startled by the sound of an ambulance she turns around and hits her eye on the car's wing mirror.
Moments later, a car drives up to the entrance of the ED and a teenage girl is pushed out onto the pavement, before the car speeds away again. David and Iain help to get the girl inside where Ethan treats her, but they're unable to resuscitate her and she dies. David offers to ring the girl's father - whom she'd wished to get there earlier before she went into VF - but he's unable to hold a long conversation as the girl's father, Peter, soon cuts off the call. In reception, Louise notices Rita with a black eye and calls her over, and she lies to her by saying that Mark did it to her and that Iain hasn't helped her at all despite knowing it's going on.
Meanwhile in the ED, a girl named Patsy is admitted following a minor vehicle crash. She calls her friend to go and visit her although they'd recently had a fall out. Elsewhere, Peter picks up a heavy metal tool from a work surface in the factory where he's working and heads out, having presumably known what the call from the ED was about. As a severe weather storm is beginning, he arrives at a man named Leo's house and attempts to get in after he opens the door. A chase ensues as Peter shouts at him for causing his daughter's death although Leo claims he isn't responsible. They eventually reach a dead-end and Leo stumbles into a storm drain in an attempt to escape. However, he trips and breaks his arm and is left hanging onto a loose rope to prevent himself from being swept down to the river.
Connie stops off at her house on the way, as they plan to stay the night at the hotel where the conference is being held. Grace isn't too bothered about her mother leaving for the night, although Connie tells Jacob that she made a big fuss over it. On their way, they're held up at a set of temporary traffic lights. With the storm worsening outside, Connie sees it as the perfect opportunity for Jacob to run through his speech with her. However, he's distracted when he sees Peter running up across the bridge. He informs them that there's a man in the storm drain, and Jacob immediately runs down to help.
Back in the ED, Patsy's friend Donna arrives with her boyfriend Theo. Despite their fall out earlier that day, Donna shows a sympathetic side to Patsy as she's being treated. Although the x-ray comes back clear, Patsy complains of pain in her neck and numbness in her hands, so Dylan orders a CT scan just to be on the safe side. Meanwhile Iain notices Rita and asks Louise what's wrong with her, and she bluntly tells him that he should know it was Mark who hit her and that Rita doesn't want to talk to him. Robyn receives some flowers from Glen and is very vocal about her appreciation of having a nice boyfriend which further upsets Rita, although Robyn is oblivious to this.
Jacob enters the storm drain and can hear Leo's calls for help although it's too dark to see him. However, Jacob is soon swept off his feet and ends up rushing down with the flow of the water leaving him and Leo in the river. Jacob manages to get him, but he soon climbs out again to run further along the bank for a better chance of saving him. Peter goes to inform Connie back at the car, and they both run down to the bridge. As Jacob stands over the cliff edge, Connie instructs him not to jump back into the water, but he defies her and jumps in anyway and is able to get hold of Leo as they rapidly travel downstream in the stormy waters.
In the ED, Patsy continues being treated, but after Rita momentarily leaves her alone, she breaks free of her neck brace which causes further injury to her. After Dylan and Rita are able to properly treat her, Donna comes to visit and just as she's about to angrily leave, Patsy tells her that Theo tried to kiss her in a desperate attempt to get rid of him so she could have Donna to herself.
Connie and Peter watch onto the river below where Jacob and Leo are being quickly swept away by the current. Peter informs her that they're heading for the waterfall, which causes Connie to panic as she runs down the bank to chase them in the heavy rain. Eventually Jacob is able to get to the bank on a section of flat rock, moments before heading over the waterfall. They take Leo back up to the road where the ambulance arrives and takes him and Peter into the ED. Although Connie tells Jacob that they were also going to head back to the ED, she tricks him and says they're still going ahead with the conference despite the dramatic events.
In the ED, Rita shows suspicion of Patsy and she eventually admits that she faked some of her symptoms to make her injuries sound worse so she got Donna's attention. When Donna returns and tells her that Theo will be out of the picture, Patsy comes clean and tells her that she was lying and that Theo never tried to kiss her which causes Donna to distance herself even more. Dylan and Rita also inform Patsy that most patients with her types of injuries make a full recovery.
In resus, Peter and Leo are both being treated. When David discovers that Peter is the girl's father from earlier, he informs him that he is needed. However, before he leaves he angily confronts Leo, blaming him for his daughter's death, claiming that he got her hooked on the drugs. Leo denies the claims, saying that he tried to stop her and is just as shocked when he hears that she's died. He soon goes into VF, but Elle and the others are able to stabilise him. David manages to convince Peter to go down to identify his daughter's body for the coroners.
Connie and Jacob arrive at the hotel for the conference, and although he's initially nervous and unable to remember his speech, Connie guides him through from the audience which builds his confidence. When it's over, they go to get a room for the night but the receptionist informs them that they're out of rooms, but could offer adjoining rooms for Connie and Jacob. She reluctantly accepts and they head upstairs. However, before long their temptation gives way and they meet at the door between the two rooms and share a passionate kiss.
Back at the ED, Rita speaks with Donna in the corridor and explains that Patsy simply made a mistake and should be given a second chance. She asks where Patsy is, and Rita confirms where he is to her delight that she's decided to give her another chance. Moments later, Rita meets Jez as he comes out of the lift and she asks him where Iain is. He says he's not really allowed to tell her, but that Iain is going to confront Mark about what he's done to Rita. She frantically runs outside to stop him and tells him the whole truth about how she hurt her eye. She also admits that it wasn't Mark who slashed her tyres, but just a patient. Louise and the others overhear and feel betrayed by her lies, and Iain shouts at her and tells her that she needs help, as well as calling her insane as she walks away in shame.
The episode was originally scheduled to air on 2 July, but was moved due to a reschedule in the TV listings. After the change, it became the first ever episode to air on 3 July. The episode featured uncredited guest appearances from Emily Carey as Grace Beauchamp, Charlie Bailey as Annie Meyer and Sydney Wade as Carmel Sims, who was previously seen at the sports day in "This Life".
It was one of the final episodes to feature scenes shown in the summer trailer released in March 2016. The scenes shown which involved Jacob jumping into a lagoon were largely anticipated by fans. A similar thing occurred with the spring trailer which included scenes showing Connie being held hostage. They were scenes which had been awaited by fans, but also aired in the latest episode.