When their stealth mode doesn't quite work out, and without help from the cowardly masters Croc and Ox, Po's plan is to walk straight towards Shen's men and surrender.
In the tower, before the huge cannon, Viper picks the locks on Tigris's chains, while Mantis has already escaped. Together thay all break free, and set about disabling the weapon. Po is distracted by Shen, allowing him to escape, and Shen, now behind numerous more cannons, fires towards the heroes destroying the tower.
The heroes survive, and Tigris instructs Po to stay behind in the jail. Po disobeys and breaks into Shen's factory, and confronts Shen again. The 5 cannot destroy the factory with an arsenal of bombs. Po is defeated again, the 5 are captured, and a cannon blast shoots Po over the river. He is rescued by the soothsayer sheep, and told of his heritage.
Shen leads a fleet of ships carrying his weapons up the river - but Po stands in their way. Shifu soon emerges to lend a hand, with Croc and Ox, and the 5 are soon rescued. The front ships are damaged and block the way out to open sea, however Shen has time to fire the biggest cannon to take out the dam. Po and the others are blown out to sea.
Courageously, Po pushes himself onto a barge and, gaining strength from inner peace, he returns every fireball sent his way - taking out Shen's ships. Everyone is amazed. Po and Shen have one last battle when Shen won't surrender, but Shen's knives cut the cannon ropes and he is crushed to death(!)
Po is the hero. Shifu is annoyed that Po achieved the secret of inner peace at a younger age. Po returns home to his dad.
Meanwhile, there is a valley of Pandas where the lead Panda, Po's biological father, is still alive - and learns of his son through a quick meditation. So, looks like we're following the Shrek plan here of a 'Kung Fu Panda 3: Meet the Parents" (with the obligatory Panda love interest), and then "Kung Fu Panda 4: Little Pandas".