Please again refer to Wikipedia:Naming conventions (films):
"Conventions: Each word in the titles of films takes an initial capital, except for articles ("a", "an", "the"), the word "to", and prepositions and conjunctions shorter than five letters (e.g., "on", "from", "and", "with"), unless they begin the title."

The overall Wikipedia:Naming conventions (capitalization)"
"In general, each word in titles of books, films, and other works take an initial capital, except for articles ("a", "an", "the"), the word "to", and prepositions and conjunctions shorter than five letters (e.g., "on", "from", "and", "with"), unless they begin the title."

This stems from both The Chicago Manual of Style and Fowler's Modern English Usage. 

As "In" is a preposition shorter than five letters, the correct caps is "in".  There may be "flexibility" in English titling, but Wiki follows the conventions cited above.