Monostable Multivibrators have only ONE stable state (hence their name: "Mono"), and produce a single output pulse when it is triggered externally. Monostable multivibrators only return back to their first original and stable state after a period of time determined by the time constant of the RC coupled circuit. Once power is applied the BJT1 is switched on and into saturation. As a result the BJT0 is switched off. This is the original zero state. When a negative ppulse is given as the input, the RC network differentiates it. The high pulse turns on BJT0 pushing it to 0 and VCE=0. This switches off the BJT1. Thus the output goes high switching on BJT0. The capacitor C0 starts charging and at one point the voltage across it which is connected to the base of BJT1. This switches it on again driving the output back to 0.