In the monostable multivibrator, the one resistive-capacitive network (C2-R3 in figure 1 of multivibrator) is replaced by a resistive network (just a resistor). The circuit can be thought as a 1/2 astable multivibrator. Q2 collector voltage is the output of the circuit (in contrast to the astable circuit, it has a perfect square waveform since the output is not loaded by the capacitor).