Simon McManus was a drug dealer friend of Holly Barton who later threw acid in Ross Barton's face.
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 gave him a chance to earn more - by "wiping the smile" from Joe's face. However, it went wrong when Simon mistakenly threw acid at Ross Barton, who had stolen one of Joe's cars. Ross was left severely burnt. Debbie confronted Simon, unaware Simon had intended to attack Joe with acid, and flew at him before revealing he got the wrong person and threatened to go to the police. Debbie, however, backed off when Simon threatened 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 kidnapped Simon, tying him up in a basement after beating him. He warned Simon never to contact Debbie and to never commit such an evil attack again. Simon was unimpressed by the intimidation attempt so Cain took it to the next level. Presenting a bottle of acid, Cain threatened to burn Simon also. As a panicked Simon began to apologise, Cain opened the bottle and threw the liquid at him revealing it to be water. Feeling as though Simon had been successfully intimidated, Cain released him. Simon began to explain how the two are quite similar, however Cain refused to believe so.