The Adventure Begins is marked as the epiphany of the Brenner Era. Being the 70th Anniversary special of The Railway Series, Brenner and crew revisited Thomas's beginning days on Sodor. There are plenty of really well-done Easter eggs to attract older fans (the classic music remixes, the "70" on Thomas's bunker) without pandering to them. It's not a total retelling of the first two books, but instead an adaptation. It takes the first two books and "revises" them in a way that better streamlines the CGI era.

However, the suspension of disbelief gets nullified whenever Henry appears in the special, as he's the biggest flaw. There are two big reasons.

1. Throughout, Henry was seen in his post-Flying Kipper shape. His new shape symbolized a critical evolution to his character both physically and psychologically. Initially, he was really vain and wouldn't leave the tunnel for really selfish reasons. Then he was shown to be really unreliable and needed special coal just to function. His accident and temporary relocation to Crewe to be reshaped helped him grow as a character. It was possibly the most important arc in Season 1 and the early books entirely. Yeah, it's been a common complaint, and there have been defenses over it (including the painstaking way of rendering the train, so calling it "laziness" won't work). Nevertheless, when both the writers and animators work so hard to execute the special and make it appeal for both the young and old equally, seeing him like this is really glaring.
   
2. Henry's characterization in the special is arguably the worst of any cast member in the Brenner Era.
    
   I can talk about the symbolism of Henry's transition from his old shape to new, but that's small compared to his contrived portrayal. The main reason why Henry trapped himself inside the tunnel was due to his severe vanity. He was afraid of the rain because he saw himself to be more important than his passengers, hence the "lovely green paint with red stripes" repetition. In both TRS and Come Out, Henry!, The Fat Controller had a really strong reason to punish him. Now, by turning his vanity into a severe case of actual aquaphobia/ombrophobia, TFC's decision to seal him in the tunnel suddenly has huge implications. Rather than punishing Henry for his vanity, he punished Henry for something beyond his control. This makes TFC appear callous and unfit for a leading man on the Island.
    
   Also, the naming of the actual tunnel suddenly has big implications. One of the biggest flaws of the entire series (yes, even the books) is how mean-spirited it can be. Each time you hear Gordon's Hill or Henry's Tunnel, you're being reminded of the negatives of that character's actions. Awdry named it "Henry's Tunnel" for geographical circumstances, but continuity is also hammered in over the head. Turning Henry's vanity into a legitimate fear makes the Island feel evil as a narrative.
    
   More importantly, Henry's ability to overcome his fear of water plays heavily in Thomas's character development. Initially, he was a cheeky, impatient misfit prone to making several big mistakes. Right after coaxing Henry out of the shed, he becomes a cheeky, impatient character, yet a little more mature and more sure of himself. Henry's flanderization has been a serious problem over the years under HiT because this isn't him, and it's still there. In Henry's Hero, his fear of bad coal was in character because it was backed up by continuity — broken continuity, but continuity nonetheless. Here, there's no such excuse.
    
   When you have to change a characterization to fit the plot, then the writing quality drops. Unfortunately, Henry's flanderization undermines the legitimacy of Thomas's epiphany and character development. Rather than becoming a sweet moment, it turns sour by being forced.