Perhaps the most tantalising piece of foreshadowing seen in The Flash so far was in the pilot, during that great (and totally out of nowhere!) Harrison Wells reveal. He opens that secret compartment in STAR Labs, gets up out of his wheelchair without any bother, and then looks rather portentously at a newspaper headline which reads "FLASH MISSING; VANISHES IN CRISIS". That's quite obviously a shout-out to DC's propensity to publish comic books with Crisis in the title, but it's most likely referencing the classic Crisis On Infinite Earths. Barry Allen does indeed go walkabout in that series, being lost in the timestream whilst using his speed powers to try and stop some big metaphysical villain from erasing the all of reality. Hopefully nothing quite so dramatic is going to happen at the end of The Flash's first season (Grant Gustin is immensely charming in the titular role, for one thing), but they're definitely hinting at something along these lines happening at some point soon.
Tom Baker is the Comics Editor at WhatCulture! He's heard all the Doctor Who jokes, but not many about Randall and Hopkirk. He also blogs at http://communibearsilostate.wordpress.com/
