Problems plague President Bartlet as he prepares to address thousands of schoolchildren, including a fire in a Russian nuclear missile silo, and lost communication with a Mars spacecraft.
Gabrielle tries to keep Grace in her life; Renee shares a dark secret with Susan; Paul's plan is revealed.
Michael seeks revenge against Whistler and Gretchen and uncovers unbelievable news; Lincoln and Michael are reunited with the gang and get an offer from a Homeland Security agent; Mahone experiences a devastating loss; T-Bag is deserted.
The team investigate the motive behind an attack on a senator's daughter. Meanwhile, Kensi's co-worker gives him a gift.