A Reason to Fight (戦う理由, Tatakau Riyuu) is the fourth episode of the Second Season of Mobile Suit Gundam 00.

Setsuna and his comrades have successfully overpowered the A-LAWS and freed Allelujah and Marina from the Federation's detention center. When Marina asks them to return her to Azadistan, they set a course back to the Middle East. But the A-LAWS have anticipated this course of action as well, and they are already deploying a powerful new mobile armor to ambush the Ptolemaios 2 in the Arabian Sea.

As the two sides prepare for battle, their members are becoming aware of their connections to the enemy. Allelujah is convinced that Soma Peries, whom he met at the detention center, is his childhood friend Marie. Kati Mannequin, meanwhile, has begun to suspect the true identity of Celestial Being's tactical forecaster. And Billy Katagiri, whose love for his old friend Sumeragi has now turned to bitter hatred, makes a fateful decision.

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