Accessing keys from a map in a SOQL Query

Using .KeySet() on a map, we will get a list of all the keys returned.

Map<Id, Master_Obj__c> masterMap = new Map<Id, Master_Obj__c>([SELECT Id, Name FROM Master_Obj__c]);
List<Detail_Obj__c> objDetailList = [Select Id, Name FROM Detail_Obj__c WHERE Master_Obj__c IN :masterMap.KeySet()]