Bane has taken over New York, he has the Foot Clan under his boot heel. After a terrible battle that the Turtles and Batman barely escaped from, Splinter was brutalized by Bane, and is in critical condition. The Turtles escape but the damage has been done.
Donatello seems to take this harder than most and he decides to test the Venom on himself and become strong enough to take down the Foot and Bane on his own.
Batman and Leonardo try to calm his Rage, but Donatello is succumbing to the effects of the Venom.