Smallville :

A summer beach party at Crater Lake takes a dramatic turn when Lois hits her head while jumping into the lake. Before Clark can save her, a mysterious swimmer named AC comes to her rescue, out-swimming Clark and leaving him baffled.

Professor Fine tells Clark that Lex is behind a covert weapons manufacturing operation. AC breaks into a LuthorCorp facility in an attempt to destroy one of Lex's dangerous experiments and is captured by Lex.