Whodunnits are the stock-in-trade of mystery and detective stories. What-done-its move into the realm of science fiction, where the key to the narrative is discovering what kind of being or process is responsible for the killing or whatever nefarious events.
In Spectral, those events are top-notch soldiers being killed by a mysterious force or entity in central Europe. An American scientist is rushed over there to provide an assessment and a solution.
Spectral does a nice job of gradually unpacking and developing this story, The supernatural explanation - highlighted in promos for the movie - turns out not to be the case. The spectral entities are deliberate creations of the Central European power that our operatives are fighting. In the end, there are elements of everything from artificial demons to the Matrix in this mix.
In an interesting departure from the Golden Age B horror movies which Spectral recalls, we have some better-than-expected star power on the screen, with James Badge Dale (The Departed) and Emily Mortimer (The Newsroom) as scientists, and Max Martini (The Unit), Clayne Crawford (Rectify), and Bruce Greenwood (everything) in various military roles.
Hey, not a bad way to spend an hour and three quarters.