“The 15:17 to Paris” starts with some of Clint Eastwood’s worst filmmaking in years and closes with some of his best. In Eastwood’s ambling look at heroism and civic duty, he revisits the famous 2015 instance in which a trio of young Americans stopped a gunman on a train from Brussels, casting the actual men involved to play themselves. The bulk of the drama leading up to the central event often has a stifling quality, marred by heavy-handed foreshadowing and bland monologues about the existential quandaries of young adulthood. However, at 89, Eastwood remains a deeply purposeful filmmaker, and “The 15:17 to Paris” clearly has a plan — it builds to a riveting showdown, with a unique kind of payoff enhanced by the authenticity of its design. It’s a fascinating gamble even when it doesn’t hold together.

Outside of that extraordinary climax, the story of Spencer Stone, Anthony Sadler,...