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Mavis continuously put Anopha Quarry's trucks in different places, much to Toby's frustration. Toby finally left Mavis to put the trucks wherever she wanted. Mavis was supported to do so by Diesel and took the trucks to a level crossing, where the trucks held her back and she got stuck on the crossing, trapping Bertie and Terence. Mavis was rescued by Toby and sent back to the quarry. She felt embarrassed after the accident and upset when she was forbidden to leave the quarry. Percy tried to knock some sense into her, but she refused to listen. She eventually made a plan with the trucks to go as far up the line as possible, but this resulted in putting Toby in danger. Mavis rescued Toby and the Fat Controller let her go further down the line occasionally.
In the fourth season, Mavis was one of the engines who took over the jobs of the engines who went to England. In the fifth season, she and Toby received help at the quarry from Stepney, who was visiting the railway.
From the sixth to eleventh seasons, Mavis worked at the Centre Island Quarry. In the sixth season she had clearly matured and was able to keep Bill and Ben in order. When Salty came to work at the quarry, he was upset that he was not working by the sea, so Mavis did her best to comfort him, while trying to stop Bill and Ben from complaining about Salty's talent with working with trucks. Mavis tried to stop Bill from being so jealous and grumpy when Ben went to get new buffers.
In the seventh season, Fergus was sent to help Mavis and the twins. Mavis got on well with Fergus and helped him out of a rockslide caused by Bill and Ben. In the eighth season, Mavis was one of the diesels given bad fuel, which resulted in her and the other diesels breaking down. Thomas collected new fuel and helped them.
When the steam engines and diesel engines were arguing, Thomas turned to Mavis for help as she was one of the few nice diesels and the two organised a meeting to sort things out. She also helped with the construction of the Sodor Airport, including taking away rubble from the wrecked tower.
In the ninth season, Mavis was involved in a collision with Percy and Toby. In the tenth season, Toby went to Mavis for help with his jobs, but Mavis was too busy. In the eleventh season, Mavis was left without fuel when Billy forgot to deliver it. In the twelfth season, Mavis was having a wash at the Sodor Slate Quarry, when she was derailed by Thomas, who was racing with Diesel.
When Spencer got on the verge of discovering Hiro, Mavis distracted him from Thomas and Percy, by guiding him to back up right underneath the operating slate hopper so he had to be taken for repairs at the Steamworks. In the thirteenth season, Mavis was awoken early by Thomas, comforted Percy who was upset about having no job, was given flowers as a surprise from Emily, who thought she was upset and attended Hiro's welcome party. She was later convinced by Thomas to shunt trucks at Brendam Docks to impress a Railway Inspector alongside Percy.
Mavis broke down one day and was taken to the Dieselworks by Salty, where she was surprised to see Percy there. In the fifteenth season, she told Spencer, who could not see properly due to heavy fog, that he was at the quarry when he thought he found the Duke and Duchess of Boxford's Summer House. She later broke down again and this time Diesel helped her. In the sixteenth season, she told Gordon to get his bufferbeam mended, even when he tried to convince her he was fine without it and helped a composer with his song alongside Thomas and some of the Steam Team.
In the eighteenth season, Diesel took her to see the sights of Sodor, while Thomas did her job at Ffarquhar Quarry.
In the nineteenth season, she got damaged by stones from the hopper and had to go to the Dieselworks to be repaired and Den was sent to the quarry to replace her. She comforted Dart when he started to miss Den and once she was fixed, she went back to the quarry.
In the third season, Mavis was arrogant and did not take advice very well. She was mostly interested in her own ideas until being brought down to earth after her embarrassing incident at the Ffarquhar crossing. Despite this, Mavis is a reliable, honest, hard-working engine and unusually for a diesel, shows respect for the steam engines. However, unlike most engines, hauling trucks makes her feel important, although she still had a lot to learn. With a little help from Toby, she now finds better ways to be really useful.
From the sixth season onwards, Mavis has matured and acts as a motherly figure to the other engines. Despite being a young engine, she still retains some of her naivety, but knows more about handling trucks ever since.
Mavis is a based on the BR Class 04 0-6-0 diesel locomotive. She is more specifically based on the BR Class 04's that worked on the Wisbech and Upwell Tramway, as they were fitted with additional cowcatchers and sideplates so they could run near public roads. 18 of these locomotives are preserved, including one "tramway" example.
Mavis is painted black with yellow hazard stripes on her front and rear. She has her name and "The Ffarquhar Quarry Co. Ltd." written in white on her cab and sideplates respectively.