Derileen "Derilee" MacGyver is part of prostitute double-act with her best friend, Sherilee, referred to as the "Gastric Bandits" by Frank. They are known for their messy eating habits and obsession with cleaning equipment which they receive as payment from Frank for their services.
Derilee is married to a man named Baxter, who she refers to as "Biddie". Baxter was violent toward Derilee and when she eventually left him, she moved in with Frank temporarily. However Baxter tracked Frank down to the school where he works while carrying a gun. Baxter didn't manage to make it to Frank but was seen by children, the school was evacuated and the police called but both Frank and Baxter had left.
Frank came home to find Baxter sitting with Stella holding a gun and it was then that he revealed that he was Derilee's husband and was there because of Frank sleeping with her. Frank attempted to downplay his relationship with Derilee, making it clear that they weren't together. Baxter then made Frank phone Derilee and claim Stella was ill and asking for her, in order to get her to the house alone.
After Derilee left Sherilee at The Jockey, the police arrived looking for Derilee. They had figured out that Baxter was the gunman when they found the local postman dead near his ice-cream van (killed by accident and without Baxter even knowing when he dropped his gun). Sherilee explained that she and Derilee were practically sisters as they had been born in the same hospital minutes apart and eventually revealed that Derilee had gone to Frank's.
With the police arriving, the situation at the house quickly turned into a hostage situation. Derilee refused to go back to Baxter, having had enough of his violence and became increasingly insulting towards him. Frank on the other hand continually tried to placate him on par with his usual cowardly ways.
The police set up camp at the Maguire house to facilitate the negotiations, repeatedly phoning the house and trying to talk Baxter down. Soon after, Monica Gallagher arrived, presumably having heard what was happening on television, she came to primp for the cameras and act worried about Stella.
Baxter's bizarre feelings about love lead to Frank mentioning that Monica was his one true love and when they heard her shouting through the wall, they arranged to swap Stella as a hostage for Monica. The police used the exchange to attempt shooting Baxter, but they missed and Stella didn't make it out, meaning Baxter was given an extra hostage.
Baxter made Frank and Monica bear their souls to each other and told Derilee that he wished they were like them, sadly mentioning that the couple had 7 children but due to Derilee being "barren", they had none. Angered by Baxter, Derilee revealed that she wasn't infertile at all, she had been pregnant by him many times but had gone for abortions. Baxter was disgusted and hit Derilee unconscious before sitting in the kitchen and making to shoot himself before thinking better of it.
While Baxter was out of the room, Monica and Frank convinced Derilee to pretend she was going back to Baxter to talk him down and get them all out of the house alive. She did this with some hesitation and told Baxter she had lied about the abortions and just wanted to hurt him, a calmer Baxter then sent the Gallaghers upstairs so he and Derilee could have some time alone.
Upstairs, Monica tried to seduce Frank but he was more worried about leaving Derilee to get raped. Downstairs, Baxter put his head to Derilee's and made to shoot them both dead but couldn't do it. As the police made to move in and Monica managed to seduce Frank, a shot was heard. It was revealed that Derilee had managed to shoot Baxter dead, freeing them all.
A few days later, Derilee and Sherilee stopped by Frank's to say goodbye. Derilee had been freed, having killed Baxter in self defense, she told Frank that it was best he didn't know where she and Sherilee were going. Derilee gives Stella one final gift and Frank sadly told Derilee that he would miss her, showing that he had real feelings for her, but she only smiled and drove off with Sherilee in a side car on a motorbike. The two women were later seen laughing happily inside a caravan.