(Note: This puzzle may require a small amount of research if you aren't familiar with the work from its theme.)
It was exactly 11 years ago -- April 14, 2008, in Sydney -- that the first Iron Man movie premiered, featuring the power-generating "arc reactor". But here, we'll be looking at different arcs and reactors.
(1) A certain period of time in the past, an arc began in a work of fiction. This arc parallels a very similar one that will occur that same period of time in the future. Nine clues and answers are completely normal; the clues' first letters spell out the period of time, and their last letters spell out the work of fiction in question.
(2) Eleven characters from said work are 'reacting' to the arc, each changing one answer during entry in a characteristic way. (Two of the new entries do not have "dictionary nature"; one is a two-word abbreviation, and the other is a specialized term.) Because of these changes, the grid makes several distinctions that are not normally made, and some cells' contents are unusual. The character changing each answer is associated with one element of a repeating sequence. For the character associated with the nth element, take the nth letter from both the start and end of the clue. These letters will spell out a description of the outcome of the past arc (which is strikingly similar to the future arc).
(3) To aid in finding these changes, eleven other clues have an extra sequence of one or more words that must be removed before solving. These sequences define the altered entries from (2) (though not in order).
It may seem that one character is missing from the 11 reacting characters from (2). The starts of each of these definitions, again in clue order, will spell out the seemingly missing character (in an appropriate way).
(4) The remaining 28 clues all have a single incorrect letter, which must be changed to another letter for the clue to be solved. These changes may not preserve the surface meaning of the clue.
The incorrect letters of both Across and Down clues will spell out a short plot summary of the events just before the first arc (which also happens to be a cryptic clue for the object involved in the second arc).
That twelfth character has made themselves known in the correct letters of the Across clues, which will form a descriptive phrase that applies three times to parts of this puzzle. These point to appropriate words describing how the two arcs are related, how they might know that, and what the arcs might lead to.
And finally, the correct letters of the Down clues will briefly describe the arc in the future (giving the solution to the aforementioned clue, as well as a detail that explains why some of those words -- and the initial reaction to that arc -- aren't as accurate as they seem).
1. English energy drink made from cola, due to be mixed with a bit of zest
7. Sports equipment, start to finish, was a sign of affection
10. Don't have a Kia: design secretes a horrible smell
12. Otter kin heard by flirtatious woman
14. "Claimant dusted me", man recalled, "with powder"
17. One leaning, honest fabulist in speech
18. Units of sound intensity initially described by Socrates
19. Nobleman Lear in rain
20. Verse in modern song by Zayn Malik
21. Appropriately crazy dare by Plato
24. Vulgar overcomplicator's advice is primarily contrived and impolite
25. Holiday local to Glasgow when Trotsky is overthrown
26. Sands a moon shape after scratching coin
27. Beer ingredient found in optimal-temperature lager, maybe
29. Need a tattletale's secret info
31. Poor quality, terrible roll of a 2
33. Setter Reagle's coat pattern is chased too soon
34. Wind cools turnover
36. Author's unsuccessful score with hot mate
37. Essentially, Gotti is the champion for Minnesota team
38. Massachusetts city camels ill-prepared to ditch element of uranium
39. Barres's position in an idiom is taking up space, capturing resistance
42. Unknown big cipher with actress Felicia
43. Burial site of Homer obscured by radio silence
46. Circle naves of Linda with much outcry
47. Vocal Kelly the astronaut's retribution
48. Bet xanthic cat ate entertaining knight
49. Big assembly from hosting pair of events
50. Duplicated author's viral joke
51. East Indian state donkey having identical outbreak of elephantiasis
1. In pairs, complain about praise
2. So, cadet fixed rag without effort
3. Alternatively, halt author
4. In the morning, Erich makes a patrotic ballad
5. Baaing odd filly might lead you to herb
6. Conversationally persuaded each mean motorcyclist delinquent
8. Character with entity for Othello and one in the past
9. Dirtbed deals for Teen Titans villain
11. Scrap of oat removed from homeless animal's plate
13. Relative is kind when assuming romance's cloying
14. Agony of trolley car #1 beset with extreme snafu
15. A conman's located, surrounded by shrieking
16. Diminish threes, dropping first half for singer Juice
22. Annoy king needing writer
23. Drow has right item from quiver
25. Book with karaoke selections
28. Drop characters written in Michelangelo's epic
30. Touch Apple product and a Kindle
31. Triply scolded babies
32. Think second-class Biblical woman would take leaf
35. Tech wizard's sons I improperly treated?
36. Police drama might trim down chaos
37. Strap for horse side or hair
38. Erstwhile car company's sheepish remarks making a comeback
40. Fate of Pompeii's entrance without tall man
41. Maneuver through 2-somes of pyramids, poisonous gas, and weaponry
42. Overturned request politician secretly votes for
44. Announced baseman Sadaharu also is gay
45. Island runner's goal (when aiming low) is celestial energy