** spoiler alert ** 
 Ah, redshirts. Any geek worth their salt knows about them -- random extras on "Star Trek" who die in almost every episode. 
 But what if the redshirts knew that their fates were coming, and tried to stop it by whatever means necessary? That's the idea behind "Redshirts: A Novel with Three Codas," John Scalzi's affectionate lampooning of sci-fi TV. It's a light, fluffy novel that touches on some philosophical ideas, but nothing too serious. 
 In the 23rd century, Ensign Andrew Dahl is assigned to the Universal Union flagship Intrepid. It's a dream job for a xenobiologist, except that some of his new shipmates are acting a little strange. 
 And soon he finds out why: the Intrepid has an incredibly high mortality rate. The captain, science officer, engineer, doctor and the handsome if dim Lieutenant Kerensky are never permanently injured, but low-ranking officers inevitably die messy, violent deaths. There's also a magic "box" that generates the answer to impossible problems. 
 The widower of one "redshirt" has already figured out the answer -- a bizarre answer that calls into question the nature of reality itself. And with Dahl scheduled for an away mission, he and his fellow endangered officers will have to take drastic steps: go back in time to a parallel world, and find a way of stopping their problems in the 21st century. 
 "Redshirts" is a fun idea for a story -- what if the disposable extras became aware of their doomedness, and tried to prevent it? There's a lot of metafictional twists and some contemplation of what reality is, but it never gets too heavy. It's a pleasantly light sci-fi story. 
 It also has some fun with the scientific improbabilities of "Star Trek" and similar shows, although the world of the Intrepid is definitely much, much dumber than most (robots with harpoons!). And Scalzi inserts some rather un-"Star Trek"-like comedy at times, such as Kerensky (and his doppelganger) being deprived of their pants. 
 However, it can get a bit confusing at times. And even now, I'm not entirely sure how you can time-travel into a reality controlling your own without that reality interfering in your plans. Scalzi... sort of explains it at the end, but also doesn't. 
 None of the characters are very deeply developed or given much backstory, but I guess that makes sense since they are meant to be "extras" who have little development. Scalzi still manages to give them depth and texture that makes them likable, even doofs like Kerensky or the acid-tongued Duvall. 
 "Redshirts" is a lightweight sci-fi adventure, but the gentle spoofery and humor make it a fun read. Just don't expect great things.