I have resolved this by adding 2 variables.
<xsl:variable name="vInternalOffenseHistoryID" xmlns:xsl="#unknown">
<xsl:value-of select="//Charge[@ID=$pChargeID]/ChargeHistory[@Stage='Plea Event'][last()]/@InternalOffenseHistoryID" />
</xsl:variable>
<xsl:variable name="vPleaEventID" xmlns:xsl="#unknown">
<xsl:value-of select="//PleaEvent[count(Plea[@InternalOffenseHistoryID=$vInternalOffenseHistoryID])>0]/@ID" />
</xsl:variable>
<xsl:value-of select="$vPleaEventID" xmlns:xsl="#unknown" />