I made it work after the following change.
<s:iterator value="event.planMap" status="mStat" >
<h4>Plan Type: <s:property value='key' /></h4>
<table id="plan">
<s:iterator value="value.details" status="stat">
<tr>
<td><s:textfield id="name" name="event.planMap['%{key}'].details[%{#stat.index}].name" /></td>
<td><s:textfield id="text" name="event.planMap['%{key}'].details[%{#stat.index}].text" /></td>
<td><s:textfield id="contact" name="event.planMap['%{key}].details[%{#stat.index}].contact" /></td>
</tr>
</s:iterator>