Langdon doesn't respond to Teabing's question of whether he's with him or against him. Instead he just focuses on the cryptex, to give himself time to think.
He figures that the only way to save himself and Sophie is to figure out the cryptex's password.
He looks out the window, and sees Britain's oldest apple tree, with its five-petaled blossoms…
Meanwhile, Teabing is reminiscing about his plot from the very beginning:
He'd started everything in motion when he discovered Saunière had summoned Langdon for a meeting. He thought the Grand Master was going to silence Langdon after he'd stumbled too close to the truth with his new manuscript.
So, he'd Silas pose as a distraught minister who'd heard a confession from a guy claiming to have killed Saunière's family and make it look like an accident. He tells him Sophie is in danger, based on this guy's confession.
That's how Teabing manipulated Saunière into meeting with Silas. Easy-peasy.
Langdon suddenly exclaims that he knows where to look on Newton's tomb, but he won't help Teabing until he releases Sophie.
Sophie's angry now. She doesn't want to leave, or let Langdon help Teabing succeed.
Now they're at a stand-off. Sophie wants Langdon to smash the cryptex. Teabing obviously doesn't want him to.
So Langdon does what anyone else would do. He throws the cryptex straight up into the air.
Teabing fires the gun but it hits the floor near Langdon's feet. He's too desperate to try and catch the keystone.
Teabing launches himself forward, catching the cryptex, but he still lands too hard. The glass vial of vinegar shatters.
Weeping, he tries to pry the cryptex open, and realizes that Langdon had already entered the password, APPLE, and removed the map. Everything isn't lost!
The missing orb is the apple that fell on Newton's head and allowed him to discover gravity.
Langdon shows Teabing that the map's safely in his breast pocket, but he's not gonna show Teabing.
And who but Fache bursts in and saves the day. He and his police retinue start to carry Teabing away in cuffs.
(He'd found them because Teabing had shown ID at the entrance to get through the metal detectors. Silly man.)
Teabing pleads with Langdon to tell him where the Grail is hidden, but Langdon smugly tells him that only the worthy find the Grail.