Simon McManus is a drug dealer friend of Holly Barton. Simon visited Holly in June 2016 to sell her some of his gear. As she didn't have much money, Holly asked to pay him when she got her wages. Upon hearing how Holly was working in a food van at festivals, and Simon suggested Holly sell drugs from the Diddy Diner on his behalf. Holly considered it but later phoned Simon so say she couldn't do it. Simon returned the following day and asked for the drugs back. Holly explained she lost them, but Simon demanded the money for them by the following day. Simon put pressure on Holly by taking her four year old step-brother Kyle Winchester whilst Holly was looking after him in the park. Holly quickly found Kyle, but Simon sent her a picture of he and Kyle together. Holly assured Simon she could get the money, and asked him to meet her in a lay by where she was running the Diddy Diner van for her sister-in-law Victoria Barton. Holly told Simon he could get his money by taking the cash box from the van, but he would need to make it look like a robbery. As Simon started to make it look like the van had been done over, Holly's brother, and Victoria's husband, Adam arrived to check up on her. Holly realised the robbery couldn't be committed at that time, as all suspicion would fall upon her and she came up with a new plan. Holly 'accidentally' left the cash box in the van cupboard for Simon to take. Victoria to the van to get the cash box, as Holly desperately tried to get hold of Simon to warn him. Simon pinned terrified Victoria to the van floor.
In 2017, Holly's mother Moira Dingle, who was struggling after Holly's death from a heroin overdose, contacted Simon and bought heroin from him. However, her ex-husband Cain Dingle found her at Holly's memorial tree with the drugs and confiscated them.
In 2018, Cain contacted Simon and paid him to help frame Joe Tate by planting drugs in Home Farm and call the police during a party. However, Joe cancelled the party abruptly, and an unsuspecting Simon was caught by Graham Foster, who tied him up and locked him in a cupboard. Graham let Simon go free the next day. Simon went to the garage to get his money from Cain, but Cain's daughter Debbie him a chance to earn more - by "wiping the smile" from Joe's face. However, it goes wrong when Simon mistakenly throws acid at Ross Barton, who had stolen one of Joe's cars. Ross was left severely burnt. Debbie confronts him, unaware Simon has intended to attack Joe with acid, and flies at him before revealing he got the wrong person and threatens to go to the police. Debbie, however, backs off when Simon threatens to make it look like it was all her fault.
As Debbie's guilt and paranoia grew, Cain took matters into his own hands. He kidnaps Simon, tying him up in a basement after beating him. He warns him never to contact Debbie and to never commit such an evil attack again, Simon seems unimpressed by the intimidation attempt so Cain takes it to the next level. Presenting a bottle of acid, Cain begins to threatens to burn Simon also. As Simon begins to apologise and panic, Cain opens the bottle and throws the liquid at him revealing it to be water. Feeling as though Simon had been successfully intimidated, Cain releases him. Simon begins to explain how the two are quite similar, however Cain refuses to believe so.