A new loop line service was planned to open soon, and The Fat Controller bought a new engine for the passenger service, a shiny three funnel stealth tender engine named Merlin. He came on Sodor when a mist rolled around and he accidently frightened James. One night Merlin rolled into the sheds where Donald and Douglas, the Scottish twins were resting for the night after a long day of goods work, "Invisibility. Off!"
"Whit a pure weird engine Douggie." Donald whispered. "Aye Donnie." Replied Douglas, "He thinks he cuid caw invisible." "Oh, hello." Merlin greeted, "My name is Merlin, who might you be." The twins looked at each other, then back to the silver engine, "Donnie an' Douggie." The twins answered "Ye'r a bufty muckle engine, how come dae ye hae three funnels?" Merlin told the twins what he told James in the morning until he went to sleep. The next morning, Donald went to Arlesburgh to collect some ballast, but he didn't expect to see Merlin there with a long line of ballast trucks.
"So you're the new engine that's going to work on the new loop line service?" Asked Rex. "Oh yes." Merlin answered, "And 'Sir Topsom Hatt' is also bringing another new engine for the goods work." Donald watched on as Merlin puffed away. "Rex." He stated, "Whin did yon Merlin git here?" "Oh, just this morning Donald." Rex said, "He's a real nice engine. He told me about his time in the war. Me, Mike, Bert, Jock & Frank were puzzled when he said 'Invisibility on'." "Me 'n' Douggie saw him dae it at th' sheds lest nicht." The number 9 engine said, "An' a', ur they mah trucks." Donald backed up to his line of Ballast trucks and set off.
Meanwhile at Tidmouth, Douglas was passing through the station where he saw Merlin with the ballast trucks. Merlin was waiting for one of the signals to turn green when he saw Douglas, but he mistook him for Donald, "Oh hello again, I thought you were at that small railway place. You must be a quick engine." "Whit urr ye talking aboot?" Douglas asked in confusion. Merlin then noticed, "Oh sorry, I forgot that you're twin engines. Which one are you again, Donald or Douglas?" "I'm Douggie." Douglas answered. Then Donald pulled in at the other platform, that's when Merlin's signal turned green, "Invisibility. On!" A large cloud of smoke filled the station, this made Douglas cough, when the cloud disappeared, the number 10 engine could see some of Merlin's trucks still beside him, he noticed that one of the couplings has snapped, this meant that Merlin was taking half of his trucks without a break van.
"Och na!" Douglas shouted "Ah need to git to that engine fleet. If he reaches Gordon's Hill 'n' they trucks begin tae break free, 'n' wi'oot a break van. There micht be a serious mishanter." Donald called a stationmaster about the problem, he allowed Douglas to take the other half of trucks to Merlin before it's too late. Douglas pulled hard, but he pulled and pulled with all his might, but he could only move at a slow pace. Donald then told the stationmaster if he can help his twin, the stationmaster asked The Fat Controller for his approval, and he accepted it. Donald coupled in front of Douglas and the two were off to get Merlin.
Some engines and people noticed Merlin was pulling a heavy line of trucks without a break van, some tried to warn him, but he mistook them for hellos and cheers, meanwhile Donald and Douglas raced past stations blaring their whistles. Merlin puffed past Wellsworth, Edward noticed the line of trucks and tried to warn the silver engine, but he disappeared before he could reply. Merlin began to up Gordon's hill and that's when the trucks began to strain, some of the couplings were old and some had the chance of breaking. The problem with ballast trucks is that they are difficult to behave, they began to whisper, "Hold back. Hold back." They pulled the train back and Merlin struggled. Then he heard two whistles coming from the bottom of the hill, then before he knew it, Merlin felt a big shove. "We made it." Donald said. "Dinnae stoap Merlin!" Douglas shouted, "We git ye!"
The three engines got over Gordon's hill and stopped at Maron. "My." Merlin said, "What were you two doing, giving me a scare with that big bump." "Sorry aboot that." Replied the twins, "We saw some o` yer trucks back at the big station 'n' we hud tae chase ye afore thare wid be an mishanter." Merlin was shocked by that, he didn't notice that he was missing some of his trucks. "Oh." The silver engine said, "I didn't know that. I'm truly sorry about that." "Tis a'richt." Donald replied. "We'll gie ye th' rest o' your trucks 'n' ye'll be on yer wey." Douglas responded. The twins uncoupled from the trucks and attached them to Merlin's set and the silver engine headed off. "Invisibility. On!" Merlin shouted as steam appeared. Donald and Douglas both smiled as they saw Merlin head off. That night, The Fat Controller commended Donald and Douglas for their deed and the twins became good friends with Merlin.