Cosmic Boy, Lightning Lad, and Saturn Girl travel back in time to help Clark destroy Brainiac, who has taken over Chloe Sullivan's body and sent Davis Bloome further down the path to becoming Doomsday. Writer Geoff Johns skillfully juggles hints of Clark and Lana's futures with who they are in the present, the way Smallville used to do so well.
Because Chloe is a character original to the show, I really believed the Legion might kill her to prevent Brianiac from wiping out civilization as we know it. In the end, Clark marshals the three heroes' talents to save Chloe—just as you'd expect of The Man of Tomorrow—reminding the Legionnaires that the good of the many becomes meaningless if they forget one person's value.
I still think the supporting characters give Clark more credit for being good and inspirational than he's actually shown:
The Clark I know would never doubt himself...
Have you been watching Smallville?