Anti-plastic

Anti-plastic was a blue liquid used by the ninth incarnation of the Doctor against the Nestene Consciousness.

The Doctor carried the liquid around in a vial in his pocket. He had originally planned to use the anti-plastic as a threat and a bargaining chip to make the Consciousness leave Earth, which the Consciousness refused to do. Discovering the Doctor's TARDIS, the Consciousness had an Auton take the Doctor's anti-plastic from him.

When Rose Tyler rescued the Doctor, she knocked over the Auton holding the vial, who fell into the Consciousness. When it did so, the substance reacted explosively with the Nestene Consciousness, which led a halt of the attempted Auton invasion. (TV: Rose)