Whose misadventured piteous overthrows doth . .

The following is taken from the prologue of Romeo and Juliet. I’d like to know why the plural noun overthrows takes the third-person singular auxiliary doth.

From forth the fatal loins of these two foes

A pair of star-crossed lovers take their life,

Whose misadventured piteous overthrows

Doth with their death bury their parents’ strife.