Sorry, I haven't actually read this specific fanfic, so I can't give a definite summary. But generally, it should be about Allen's encounter with the 14th, which might include some kind of conflict or discovery.
Since I haven't read the 'D Gray - Man Allen Meets the 14' early fanfic, I can't give a detailed summary. But from what the title suggests, it's likely that the story centers around Allen's meeting with the 14th. There may be drama, mystery, and a lot of exploration of the relationship between Allen and this mysterious 14th entity within the context of the D Gray - Man universe.
One possible aspect of an early fanfic where Allen meets Neah could be the exploration of their complex relationship. Maybe it delves into how Allen initially reacts to Neah's presence, whether with suspicion or a strange sense of familiarity.
Well, in the fanfic, Allen might have a very intense encounter with the 14th. Maybe there would be a battle of wills between them, as Allen tries to resist the influence of the 14th. It could also involve some revelations about Allen's past or the nature of his Innocence that are related to the 14th.
Perhaps Allen and Neah would have an intense verbal exchange. Allen might question Neah's intentions right away, given Neah's somewhat mysterious nature in the D Gray - Man series.
In this brainwashed Allen fanfic, it could also be that the brainwashing is part of a larger scheme. The Millennium Earl might be trying to use Allen as a weapon against the Black Order. Since Allen has the Innocence and his special powers, if he's under the Earl's control, he could cause a great deal of damage. The fanfic could then detail the battles that occur as a result, with the Black Order having to fight Allen while also trying to save him. They might have to find some long - lost knowledge or a special item to reverse the brainwashing process.
In some fanfics, Allen might be depicted as dead, but it really depends on the specific plot and the author's creativity. Fanfic writers often take liberties with the original story, so there could be various interpretations.
One possibility is that the fanfic focuses on the relationships Allen has in this new state. His friends might be confused or worried about him. For example, Lenalee could be trying to reach out to him, but Allen is pushing her away because he doesn't want to put her in danger. And the Order might be divided on how to handle Allen now that he's different. They could see him as a threat or as a potential weapon against the Millennium Earl.
Perhaps the Earl discovers Allen when he is in a vulnerable state. The fanfic might explore how the Earl's presence affects Allen's development. It could also show Allen's internal struggle between the kindness the Earl might show him and his loyalty to the Black Order. It could be a story full of psychological drama as Allen tries to figure out the Earl's true intentions.
There are some fanfics that focus on Allen Earl's growth in power. They might describe his training sessions in more detail, and how he overcomes his weaknesses. Maybe he discovers a new form of Innocence or a unique way to use his existing powers. These fanfics often add exciting new elements to his character development.
Isolation is also a theme. Allen is often alone in his struggles, being the host of the 14th Noah. Fanfics may depict his feelings of isolation from both the Exorcist side and the Noah side, and how he copes with being different from everyone else.
One plot idea could be that the Earl is hiding Allen in a parallel dimension only he can access. Allen has to figure out a way to escape while the Earl tries to convince him to join his side. Another idea is that the Earl disguises Allen as one of his Akuma, and Allen has to maintain the ruse while secretly trying to undermine the Earl's plans from within. Maybe the Earl hides Allen's Innocence in a way that Allen has to go on a quest to find it again, all while being under the Earl's watchful eye.