Trek Time Travel

Star Trek, like most sci-fi franchises, has been known to dip into the well of time travels stories, on occasion. It's almost expected for every television series to travel back to the viewers' "present day" at least once in its life. But, on the whole, how successful have time travel stories been? Do they "make sense"? Do they need to? Does introducing the knowledge of time travel "break" the Star Trek universe? Join the trio as we step into the TARDIS, er, slingshot around the sun on an adventure in time and space ... um, the final frontier.